Lecture 10: Atmospheric pressure and wind (Ch 4) map discussion wrap up Ch 4

SIGNIFICANT WEATHER DISCUSSION FOR THE CANADIAN PRAIRIES ISSUED BY THE PRAIRIE AND ARCTIC STORM PREDICTION CENTRE OF ENVIRONMENT CANADA AT 2:00 PM CDT THURSDAY SEPTEMBER NO WATCHES/WARNINGS IN EFFECT. DISCUSSION...CANADIAN PRAIRIES REMAIN IN NORTHWESTERLY FLOW ALOFT THRU FRIDAY. SURFACE LO JUST NORTH OF FORT SMITH WITH TROF AND WARM FRONT EXTENDING SEWDS THRU SK. EXTENSIVE AREA OF RAIN AHEAD OF THIS SYSTEM…. WEST OF SYSTEM LGT-MDT WLY SFC FLO WITH NEAR TO ABV NORMAL TEMPS. Let’s interpret these comments in relation to the 12Z charts Let’s interpret these comments in relation to the 12Z charts

500 mb analysis, 12Z Thurs Sep28 (mild, cloudy morning) NW wind over W. Canada weak ridge upstream troughs in Gulf Alaska and E. Canada weak trough in E. central Ab winds follow height contours test the Geostrophic law (see next page) Stony Plain thickness 558 dam L

 h=60 m (6 dam) over distance (“L1”) which is:  x = 2.2 x 10 5 [m] (this value subject to about 10% measurement error with your ruler) L1 L2 measure L1, L2 with your ruler true distance of L2 is 11 x 111,000 m therefore true length of L1 is  x = 11 x 111,000 x (L1/L2) assume station is at latitude 56 o, so f = 2  sin (56) = 1.21 x [sec] Test formula against this observation I get 22 m s -1, which is a bit low compared to the observed 30 m s -1. I probably over- estimated L1 I get 22 m s -1, which is a bit low compared to the observed 30 m s -1. I probably over- estimated L1

Thickness of the layer 1000 mb mb The height interval between that height where p = 1000 mb and that height where p = 500 mb is called the (1000 mb mb) thickness and is related to the average temperature of that atmospheric layer. Thickness is quoted in [dam]. 2 dam 1 o C An increase (decrease) in thickness of 2 dam corresponds to warming (cooling) by 1 o C

sfc low N of Ft. Smith sfc trough drapes along E border rain and falling press in SW Sask rising p in C. Ab in wake of trough warmer in Ab than Sask (front?) Fort Smith, NWT

much warmer at Stony Plain than Le Pas (Manitoba) steepest  T seen in SW Sask (signature of front) distinct 850 mb trough the 500 mb trough lies upstream from the 850 & sfc troughs 850 mb analysis, 12Z Sep28 0oC0oC0oC0oC tight temp grad. note how the 0 o C isotherm wraps far NW then plunges south over eastern Canada… common scenario

westward edge of the deep cloud in Sask roughly aligns with 850 trough axis and baroclinic zone ie. clearing west of the low level trough such that E and C. Ab clear Nb! The vis is at 1430Z

Please plot the given sounding on the skew-T/log-P diagram

500 mb analysis, 12Z Wed Sep27 (classic Edmonton fall day; sunny all day, high 17 o C) massive ridge over B.C. Stony Plain thickness 552 dam