VOLUME 5, NUMBER 2, MAY 1980

A SNOW INDEX USING 200MB WARM ADVECTION

Billie J. CookWeather Service Forecast Office

Fort Worth, Texas

!

ABSTRACT

Warm advection at 200mb is used toforecast the amount of snowfall inthe ensuing 24 hours. The type ofadvection at 700mb is used to modifythe forecast. A test of the methodwas made from October 15, 1976through March 1, 1977 with favorableresults.

1. INTRODUCTION

Most heavy snow forecast techniquessuffer from being too complicated,are too time-consuming, or fail toprovide a forecast of both maximumamount and area. The methodpresented here overcomes some ofthese difficulties. An initialestimate (or "snow index") is madefrom a calculation using the 200mbchart. Adjustments are made based onadvection at 700mb and a few simpleguidelines. Application of thetechnique requires only a fewminutes.

2 . BACKGROUND

Most snow forecasting schemes use thelocation of low pressure centers(surface or aloft), or vorticitymaxima. See Goree and Younkin (1),Spar et al (2), Hanks (3), Browne andYounkin (4), and Harms (5). Ostby (6)suggested the use of Model OutputStatistics (MOS). Goree and Younkinidentify prediction of heavy snow asone of the most important functionsof the National Weather Service, butstate that " ••. on the average, only amodest degree of success can beclaimed."

Most of these approaches make noattempt at forecasting the amount ofsnowfall. Harms and Hanks make suchan effort but if the area of concernis not on the forecast path ofmaximum snowfall, then the amount isonly an estimate based on the

location in relation to the maximum.All except Ostby relate heavysnowfall to the track of the surfacecyclone or its upper features.

3. TYPE OF PRECIPITATION

The type of precipitation must bedetermined before snowfall amountsare to be successfully forecast. Theoperational POF forecasts based onMOS incorporated the better featuresof Wagner (7) and Younkin (8). TheMOS POF progs are quite good, and ifthey forecast snow any precipitationis likely to be snow. However, a fewimportant snowfalls have occurredwhen the POF was less than 50percent. Cooling caused byprecipitation is apparently notentirely accounted for in the POFprogs. Wexler et al (9) show thatwith a moist adiabatic lapse rate,surface air at a temperature of 40Fcan be cooled to 32F by .38 inch ofprecipitation. Evaporation canmodify the lower atmosphere in ashort time. The wet-bulb temperaturecurve from soundings may be used todetermine the type of precipitation.A simple rule is that precipitationwill be snow or change to snowwhenever the wet-bulb temperaturecurve goes below freezing within 2500feet of the surface and does not goabove freezing at a higher level.Significant precipitation should beexpected (about .25 inch, waterequivalent) .

In January 1974, the type ofprecipitation was compared to theheight of the 200mb surface over thecontiguous 48 states. Snow accountedfor 81 percent of the precipitationarea between the 164 (Note: Contourvalues in decameters with initial "1"omitted - e.g., 164 = 11640 meters)and 176 contours at 200mb. The areawas determined by one degree latitude

longitude "squares", where anyreport of precipitation within a

29

B. The heaviest snowfall in theensuing 24 hours occurs near thecoldest air at 200mb that isdownstream from the warm pocket,provided of course that the column ofair is cold enough for snow. This isusually immediately downstream fromthe strongest indicated warmadvection at the initial time (T+O).

The thermal field at 200mb can thenbe a measure of the potentialvertical motion over a particularlocation. For example, if the 200mbtemperature at some location is -53Cand the warmest 200mb temperatureupstream is -48C, then the potentialwarming at 200mb as a result of theupstream system is only 5C and thevertical motion as it approaches willbe only moderate. On the other hand,if the 200mb t empe r a t ur e at somel oc a t i o n is -GOC and the warmesttemperature upstream is -45C, thenthe potential warming at 200mb is 15Cand the vertical motion will bestrong, provided the change occurs ina reasonably short time. Experiencehas shown a reasonable advectivespeed for the warm pocket to be about14 degrees at latitude (degrees lat)in 24 hours, or an average of 35knots. Examination of the 200mbcharts over several winter seasonsreveals some interesting relations:

NATIONAL WEATHER DIGESTsquare was assumed to be valid forthe entire square. Rain occurred in11 percent and freezing rain in 8percent of the area. Between the 176and 188 contours snow accounted for36 percent of the area, rain 51percent, freezing rain 12 percent,and ice pellets 1 percent.Experience gained since 1974 suggeststhe 176 contour as the 50 percentprobability line for low elevationswith average surface pressures (1005to 1015mb) • For each 3000 ftincrease in elevation or for highersurface pressures the next highercon tour (188) is used. Also forlower pressures or in the immediatecoastal sections the 50 percentprobability is one contour lower(164) •

4. DEVELOPMENT OF TECHNIQUE

It should be emphasized that thesecomments are only for wintertimeconditions (mid-October throughFebruary). This technique reliesprimarily on the 200mb chart withminor checks from the 700mb andsurface charts. First a brief wordabout the 200mb chart. Warm airnormally occurs in 200mb troughs andcold air in the ridges. Temperaturesare -40C to -45C in strong troughsand are -65C or colder in strongridges. Temperatures remain in the-50s with weaker systems. Thus thethermal pattern at 200mb reflects thestrength of the system occurring atlower levels.

A. Cloudinessto increaseadvectionindicated cold

and precipitation tendunder indicated warm

and decrease underadvection at 200mb.

The indicated advection pattern at200mb gives important clues to thevertical motion fields and, moreimportantly, it indicates thepotential vertical motion field.Austin (10) says, "The strong(temperature) changes which occur inthe upper atmosphere may beconsidered as arising from verticalmotion fields which accompany thesea-level pressure changes and whicharise from such low-level processesas heating and cooling. Anon-uniform field of vertical motioncreates strong advective fields inthe lower stratosphere as a result ofthe abrupt change in the verticaltemperature gradient near thetropopause."

30

C. Vorticity maxima at 500mb tend tomove toward the 200mb cold pockets;e.g., if a 200mb cold pocket developsnortheast of a 500mb low, the lowwill recurve in 24 hours. Note thatthe 200mb warm pocket almost alwayscoincides with the 500mb vorticitymaximum, particularly in welldeveloped systems. Generally thedirection of movement of a vorticitymaximum at T+12 to T+36 hours will beparallel to a line connecting the200mb warm and cold pockets at T+O.On occasions when the 200mb warmpocket is ill-defined or shows as anelongated warm sector, then the 500mbvorticity maximum is used as the200mb warm pocket. Forecastcontinued digging if the cold pocket

5. THE INDEX

index, or the averagein inches, for the24 hours will beone-half the indicated

(C degrees) at 200mbcolumn of air is coldsnow. The maximumadvection, limited to

degrees lat) upstreamused for a 24-hourthe indicated warm

VOLUME 5, NUMBER 2, MAY 1980the solid arrow, utah to Pennsyl­vania, suggested a northeastwardmovement after 24 hours. Note thatthe track of the 500mb low wasessentially as suggested by the solidarrows.

Jacobson et al (11) used the coldesttemperature at 500mb and the windflow to forecast surface development.Warm air at 200mb reflects cold air

at 500mb. If the storm is not strongenough to affect the 200mbtemperature, heavy snow usually doesnot occur. Jacobson said it was notnecessary to worry about the moisturein the developing storms. If thedynamics are strong, the moisturewill find its way into the storm.This appears to be true. Initiallydry conditions serve only to delayprecipitation a few hours.

A. Thesnowfall,followingapproximatelywarm advectionprovided theenough forindicated warm840nmi (or 15should beforecast. If

Oliver and Oliver (12) and manyothers concur that cold advection inthe mid-troposhere (700mb) indicatesdownward motion and warm advectionupward motion. The 700mb advectionis used to adjust the forecastderived from the 200mb chart.

D. Cold air (-60C or colder) usuallydevelops northwest of a short wavetrough before it deepens. The coldair is frequently over the Gulf ofAlaska as the short wave moves on thewest coast.

E. The height contours at 200mb arefairly conservative. Extrapolationof the past 24-hour changes for thenext 24 hours usually works well.Generally the contours may be used insitu or with only a small forecastchange, usually less than 120 meters(one contour interval).

The 200mb temperature pattern seemsto give clues about 24 hours inadvance of the recurvature. Note thesolid arrow drawn from the 12GMT200mb chart on the 9th indicated acontinued digging through 36 hours.But by OOGMT on the 10th, the solidarrow had turned nearly east-westreaching from Oregon to WestVirginia. This suggested the 500mblow would begin to turn eastward inabout 24 hours. By 12GMT on the 10th

At OOGMT March 11 the LFM forecast aclosed low over Northeast New Mexico- an excellent forecast, while thebarotropic did rather poorly. In thenext three forecast periods the LFMcenter was too far south while thebarotropic center was equally too farnorth. Following these periods bothtypes of progs improved. The LFM isknown to be better than thebarotropic in the development stagesbut is slow in forecastingrecurvature.

On Figure lb the solid arrows connectthe 500mb vorticity maxima with thecoldest 200mb air downstream. Thehead of the arrow marks the centralpoint of the coldest 200mbtemperature. Note the rotation ofthe arrow heads with time.

is southeast of the 500mb low andrecurvature if the cold pocket isnortheast of the 500mb low. Thistechnique should not be used with twotypes of patterns. They are: largescale cyclonic flow over NorthAmerica with short waves movingrapidly through, and cutoff lows inthe southwestern U.S. that haveremained nearly stationary for theprevious 24 hours.The storm that caused blizzardconditions through Eastern Coloradoand Northwestern Kansas in March of1977 is traced in Figures la and lb.The crosses connected with the dashedlines depict the 500mb vorticitymaxima. On Figure la points marked"B" indicate the 36-hour forecastbarotropic vorticity maxima, whilepoints marked "L" indicate thepositions of the 36-hour LFMvorticity maxima. Dotted linesconnect each "B" and "L" to the crosscorresponding to the verifying time.

31

NATIONAL WEATHER DIGEST

""_.P~

.,T:~\}~i.... .-r,.,:;,

.-t-'-:;-- - . .....f!.",~.:~~'~

~==J:=~~~fj~ffJ~\f· . lr· . · . ·" <>.;;" •r :'>'.'\ '.»: -~. "~;--- -;1,'

1·r-~~~~·· .'

\ or """--.1 r-~ \.~~\- Figure lb. Crosses same as le , Sol id,I,' arrows connect 500mb vorticity maxima with. . coldest 200mb air downstream. Head of arrow isI,,:.. .. " positioned at center of 200mb cold pocket.

("';v y-1-,-~ , x., \\i: \--:; : :. B~ . \.~/ '< \ !~ ; '-=1":' - --,--'r~;o''''''''\ 6'' /I::r---· ! a . \ " V 13' 12Z'T t\ - .--J:(J , . ' ..

\' ;1 , : .: :.,."r:: .......;-...... \I'~ . .

, ", : ' 'ti f i: \ ",.I ~'" ., f I : : -.. , ....,rt 1 0.0 . J:>~ '. ,• : ~O-l2Z : :. ./~1 2- 1 2~1 \ J A

~' : .;." , L j'-r .>:..:

~ '.ll. ...._- 11- 001 , I vf., : : r.:»>il . - <-2 '+-1- ~? '

I~ . depict observed vortici ty max ima. Points marked

.«l "B" are 36- hour Ba rot ropic foreca st vorti c it y, ~ max ima. Points marked "L" show 36-h our LFM

. .__ forecast vor t icity max ima . Dotted I ines connect --0--"""'­the "Bs" and " Ls" t o th e subsequent ly observedposition of the vort ic i t y maxima.

\ / !"fl~ (/t~j "I,,:..~ --r..L'·r"'~_ ,J

' / .~ .-I--~-"

32

VOLUME 5, NUMBER 2, MAY 1980

33

NATIONAL WEATHER DIGESTadvection extends less than 6 degreeslat (360nm) upstream from theforecast area, the precipitation isusually of short duration. Generallythe indicated warm advection ismeasured from the warmest temperatureto the coldest temperature along the200mb contours.

B. If cold advection is observedwithin 8 degrees lat of the forecastarea at 700mb, the snowfall forecastin paragraph A should be divided by2. The index then becomes 1/4 theindicated warm advection at 200mb.The 700mb temperature 8 degrees latupstream shoUld be compared with the700mb temperature over the forecastarea. If the upstream temperature iscolder than the forecast areatemperature, then cold advection i sassumed and the 1/4 rule is used;otherwise the 1/2 rule is used.

C. Maximum snowfall in the ensuing24 hours occurs near the coldest200mb temperature that is downstream(parallel to contours) from thewarmest 200mb temperature. It willfrequently be within or near the areaoutlined by Ostby (6). Heavy snowbands are almost always parallel tothe T+O 200mb contours or cross themat a slight angle from higher tolower values as the snow progresses,usually northeastward. This is thenormal turning of the 200mb contourpattern as the storm movesnortheastward.

of the area of coldest 200mbtemperature, or limited to 14 degreeslat downstream from the warmest 200mbair for a 24-hour forecast (21degrees lat for a 36-hour forecast).

The snowfall of December 6-7, 1976 ispresented as an example of an averagesnowfall event (Figure 2). It wasselected because a forecast was madeshortly after receivi ng the 200mbchart for OOGMT December 6, 1976.The dotted line outlines a forecastarea for an average of 2-1/2 inchesof snowfall to verify at l2GMTDecember 7, 1976. Warm advection wasind icated at 700mb using the criteriadescribed earlier. Five degrees warmadvection was indicated at 200mb fromColorado to the forecast area givinga snow index of 2-1/2. The area wasselected as being in a cold 200mbtemperature zone which was downstreamfrom the warmest 200mb temperature.It was selected in an area where thePOF was 50 percent or higher forthree subsequent l2-hour forecastperiods. The western end of the areawas placed slightly downstream fromthe significant warm advection andthe eastern end was placed far enoughto cover most of the cold 200mbtemperature. Twenty-one latitudedegrees downstream from the warmpocket allows for a 35 knot averagemovement through the 36-hour forecastperiod. The width of the forecastarea was selected as 120 meters onthe 200mb chart.

6. TEST OF TECHNIQUE

The observed 24-hour snowfall area at12GMT December 7, 1976 is shown bythe dash-dot line. It includes allreports of more than one inch. Theaverage in the area was 2-1/2 inches,with a maximum of 4 inches. Thisforecast was considerably better thanaverage and took less than 5 minutesto prepare. The surface chart forOOGMT December 6, 1976 is shown inFigure 3. Twenty-four hours laterthe occluded front extended from aweak low north of the Great Lakesthrough lower Michigan to anotherweak low in southern Louisiana.

The snow cover as indicated on NAFAX

(e.g., the Greatmountains) can

of snowfall

D. Local effectsLakes, hi lls andaffect the amountconsiderably.

E. Location of the area of heavysnow is usually the main problem. Asnoted previously the heaviest snow isalmost always immediately downstreamfrom the maximum indicated warmadvection on the 200mb chart. Thewidth of the heavy snow band isfrequently about the same as thedistance between adjacent contours onthe 200mb chart. The southwest endof the heavy snow band should beplaced just downstream from themaximum indicated warm advection.The northeastern end of the forecastarea should be near the eastern side

34

map N73 was used to determine theamount of snowfall for each day fromOctober 15, 1976 to March 1, 1977 bysubtracting the snow depth for theprevious day from that for "today"over the 48 states and adjacent areasof southern Canada. The liquidprecipitation indicated on NAFAX N81was a lso used to "confirm" thesnowfall. This method has obviousweaknesses, notably the assumptionthat each snowfall began after 12GMTand ended by 12GMT the next day.Settling and melting are notaccounted for and surely some largeamounts are omitted from the snowcover chart. Nevertheless, the sizeof the sample and the cross check ingwith the liquid precipitation shouldhave given a fair estimate of theaverage snowfall. Mountain topreports and isolated reports, as wellas reports obviously influenced bythe Great Lakes, were not used inthis test. A minimum of threereports over an area of at least 3000square miles was required forconsideration as a snow event.

The highest index obtainable in theU.S. or adjacent section of Canadawas derived from the 12GMT chartseach day during the test period andcompared to the snowfall reported 24hours later. On 58 days no index wasrecorded due ~o a lack of significant200mb warm advection; no significantsnow was reported on the followingdays. All other indices andsubsequent snowfall are listed i nTable 1 and compared in Figure 4.The abscissa is the snow index (orforecast snowfall) and the ordinateis the observed average snowfall.The plotted numbers represent themaximum snowfall reported. A perfectfo recast of the average snowfallwould fallon the heavy line. Thel ighter lines above and belowrepresent a depa rture of one inch.There were a few occasions where oneor more of the charts were miss ing,and more than one snow e ve n t wasr eco r ded o n a few charts. Of the 80s now events, 64 percent were withi no ne inch of t he forecast amount.

Here are some of the known weaknessesof this technique:

VOLUME 5, NUMBER 2, MAY 19801. Heavy snow should notbe forecast beneath astrong confluent 200mbflow. The heavy snow willusually be south of theconfluent zone.

2. Heavy snow should notbe forecast south of thesurface low. Occasionallythe 200mb war m pocket andcold pocket wil l appearsouth of a well-developedsurface low. Under theseconditions l ittle or noprecipitation occurs, asthe surface low . usuallymoves rapidly eastward,parallel to the 200mbcontours . This situationoccurs most frequently wi t hnorthwest flow aloft.

3. Under northwest flowaloft the systems can moverapidly and some snow mayfall with indicated coldadvection at 200mb. Thiswas observed five timesduring the test period.Maximum snowfall for theseevents was two inches.Most stations reported 1inch.

4. Occasionally a weaktrough-ridge couplet at200mb will accompany a500mb low and signal a turneastward too early. Thispattern has very weak f l owbetween the trough andridge and the half-wavelength is usually l es s than10 degrees lat.

5. This forecast scheme isknown to be unreliable inspr ing and early fall. Itshould not be us ed beforeOctober 15 or after March10. Late spring s t o r mshave been observed withlittle temperatureadvection at 200mb andothers have occurre~ underthe strongest 200mb warmadvection rather than thecoldest air, as used inwinter.

35

The 200mb chart for 12GMT January 29(Figure 5) indicated the strongestwarm advection over southern Arizona.

Maximum precipitation was reportedover southern Arizona at l2GMTJanuary 30. A strong confluent zonewas noted from the four corners tothe Texas Panhandle. The coldest airwas from southeastern New Mexicoacross North Texas, a potentialthreat zone for significantprecipitation.

NATIONAL WEATHER DIGESTA series of 200mb charts is presented(Figures 5 through 8) to illustratethe sequence of events in connectionwith heavy snow which fell over thenorthern portions of Texas andLouisiana in late January 1977, asdelineated by the dash-dot line inFigure 7. Cold air had pushedsouthward with the surface cold frontwell into the Gulf of Mexico by l2GMTJanuary 29, 1977. A 500 mb closedlow off the southern California coastwas forecast to move eastward whileopening into a trough by l2GMTJanuary 31, 1977.

By OOGMTtemperatures

January 30warmer than

(Figure 6)-50C were

Table 1

IlIi&lO~18·76

10-20-76

10-21-76

10-22-76

10·23·76

10-26-76

10-31-76

10-31-76

11-3-76

11-5-76

11-6-76

11-11-76

11-12-76

11-13-76

11-13-76

11-14·76

11-15-76

11-11-76

11-19·76

11-24-76

11-25-76

11-26-76

11-27-76

11-28-76

11-29-76

11-29-76

12-1-76

12-4-76

12-5·76

12-6'·76

12-6-76

12-7-76

12-7-76

12-8-76

12·8·76

12-11-76

12-16-76

12-20- 76

12-25·76

12-26- 76

36

Index-22

2.S

2.S

I

2.S

S

'.S3

•I.S

a••3

3

I.S

I.S

I

2

3

3

2.S

2.S

1

oS.S

2

•2.S

•2.S

NEG

I.S

L5

I.S

2

3

3.S

AverageSnowfall-

2

•S

S

I

2

2

2

••3

2

o3

I

I

2

2

3

2

I

I

oI

3

2.S

2

I

I

I

I

oI

2

3

•

ObservedMaxillur:lSn~fall

2

•••2

•11

S

I

S

2

S

•10

S

3

o

•I

I

S

S

•S

2

3

a3

••2

3

I

2

3

a3

2

••

Location

ERN SD-MN

8RN QUE

SRN QUE

MT

ERN SD-)Qi

ERN CO-WRN K5

SERN QUE

VT-NIl

NIl

NY·NlI

LWR HI

KY-WV

SERM mt-WRN TX

N CNTRL TX

OWRN AR

WV

crSRN ONT

NY-MA

NERN MT

MT-NY

WRN I<S

NN

SRN IL-NH

NY-ME

WRN ND

NY-NH Snow at Buffalo

NY

SRN lIN

HE Kl-SRN LWR MI

LWR HI-SERN ONT

NO

SERH SO-NERN MO

NO

NY·VT

VT

HE OF ME

NY·NIl

WV-RI

ME

Date-17-27-76

12-28-761-1-77

1-2-77

1-2-77

1-3-77

1-3-77

1-4-77

1-4-77

1-7-77

1-8-77

1-9-77

1-9-17

1-11-77

1-12-77

1-13-77

1-14-77

l-lS-77

1-17-77

1-18-77

1-18-77

1-19-77

1-20-77

1·22-77

1-23-77

1-24-77

1-25-77

1-28-77

1-30-77

1-30-77

2-2-77

2<5-77

2-13-77

2-19-77

2-20-77

2-22-77

2-23-77

2-25-77

2-26-77

2-27-77

Index-21.5

3

2.5

3.S

•2.5

'.54

3

i.s

2.5

NEG

NEG

I

oNEG

2.5

NEG

S

3

2.S

2.5

3

5

•• 5

2.5

2.5

3.5

1.5

6

2.5

?S

2

1.5

2

Av('r_'J~c

Sno"t;111-2

2

1

3

3

4

6

3

2.5

I

I

I

2

2

2

ObservedMaximw!Snowfall-3

•3

3

5

5

•6

••5

13

?

•2

3

32

4

3

2

5

••1

3

•4

S,•5

4

Location­loIN-HI

KY

TX PNHDL-SRN KS

$RN MO-WRN lY

OR-ID

NV

NY

NRN NO

WV

NA-METX PNHDL

&RN OK-SRN HO-NRN AR

IL-OH

ID PNHOL

NO

IN-OH

PA

ERN MJ-IL

NS

AL·GA

SERN NO-NWRH IA

NRN IL

KY

NERN I(S

J«)-KY

NY

NE

PA.-NH

N CNTRL TX

NERN TJ(-SWRN AR

TX PNHDl

VT-ME

MEKY-WV

MA-NH

WY-SWRN SO

ERN SO-MN

TX PNHOL

SERN KS-LWR HI

LWR HI

analyzed across southern and lowerCalifornia while the -GOC isothermcontinued from eastern New Mexicoeastward across the southern states.The strong confluent zone was acrossnorthern New Mexico, the TexasPanhandle, and northern Oklahoma.Strong warm advection outside theconfluent zone was occurring acrossnorthern Mexico into southeastern NewMexico and was forecast to continueeastward across North Texas. The700mb temperature 8 degrees latupstream was 5C warmer than thatobserved over North Texas. The snowindex was therefore computed as 10degrees warm advection at 200mb,divided by 2 to equal 5 (inches ofsnowfall) • Observations and thick-

VOLUME 5, NUMSER 2, MAY 1980ness forecasts indicated the snowwould be north of the 200 contour onthe 200mb chart. Radar detectedscattered light rain over extremeWest Texas beginning at 05GMT.Precipitation changed to snow withina few hours.

By l2GMT January 30 (Figure 7), thestrongest indicated warm advection at200mb was l7C over northeast Texas.Warm advection was continuing at700mb, the index became 8-1/2 inches.

The strong confluent zone continuedov~r northern Oklahoma and, to alesser extent, over northernArkansas. The first indication of asurface low was a reported 1017mbpressure near Brownsville, 330nm

8

6

10

7

6

6

5.

o.5

'.

.4

SNOW INDEX

5..

8

6

7

Figure 4. Graph of snow index vs. snowfal I.Abscissa Is snow index, or forecast snowfall InInches; ordinate is observed average snowfal I in24 hours. Plotted points sized proportional tonumber of cases. Numbers plotted for each pointrepresent reported maxima.

37

NATIONAL WEATHER DIGEST

Figure 5.

2'l.._--pJ;="",..,:,J.,;....:..--::~r~

Figure 6. 200mb

38

VOLUME 5, NUMBER 2, MAY 1980

.'"'..?

39