National Weather Service United States Department of Commerce

Heat Builds in the West; Extremely Critical Fire Weather in the Northwest

A heat wave over the Southwest will intensify this week, peaking and expanding into the Intermountain West and northern High Plains this weekend. Critical to extremely critical fire weather conditions are forecast from the Northwest and the northern Great Basin this weekend. Particularly Dangerous Situation Red Flag Warnings have been issued. Read More >

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260801_1200
Using the MB925 to MB850 layer.
Assumes greens are orthogonal to 270 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Sat 08/01 12Z 0.15 2304  85  69  14 0.00
Sat 08/01 13Z 0.14 2105  86  61  14 0.00
Sat 08/01 14Z 0.22 2206  87  50  14 0.00
Sat 08/01 15Z 0.45 2105  86  36  14 0.00
Sat 08/01 16Z 1.07 2106  82  29  14 0.00
Sat 08/01 17Z 1.55 2206  77  31  15 0.00
Sat 08/01 18Z 1.94 2306  75  24  15 0.00
Sat 08/01 19Z 1.66 2205  72  18  15 0.00
Sat 08/01 20Z 1.49 2105  70  16  16 0.00
Sat 08/01 21Z 0.79 2105  69  16  16 0.00
Sat 08/01 22Z 0.36 2006  68  17  16 0.00
Sat 08/01 23Z 0.21 2007  67  19  16 0.00
Sun 08/02 00Z 0.23 2009  67  23  16 0.00
Sun 08/02 01Z 0.37 2011  71  33  16 0.00
Sun 08/02 02Z 0.46 2015  73  39  15 0.00
Sun 08/02 03Z 0.49 2118  75  47  15 0.00
Sun 08/02 04Z 0.55 2119  78  48  15 0.00
Sun 08/02 05Z 0.55 2120  82  37  15 0.00
Sun 08/02 06Z 0.54 2121  82  48  15 0.00
Sun 08/02 07Z 0.58 2221  79  51  15 0.00
Sun 08/02 08Z 0.60 2222  79  37  15 0.00
Sun 08/02 09Z 0.61 2222  78  40  15 0.00
Sun 08/02 10Z 0.65 2222  82  43  15 0.00
Sun 08/02 11Z 0.61 2221  83  40  14 0.00
Sun 08/02 12Z 0.52 2219  85  42  14 0.00
Sun 08/02 13Z 0.50 2219  88  40  14 0.00
Sun 08/02 14Z 0.45 2118  87  39  14 0.00
Sun 08/02 15Z 0.45 2016  89  30  14 0.00
Sun 08/02 16Z 0.39 1916  90  34  14 0.00
Sun 08/02 17Z 0.42 1918  90  30  14 0.01
Sun 08/02 18Z 0.63 1920  87  34  14 0.01
Sun 08/02 19Z 0.65 1922  85  27  15 0.01
Sun 08/02 20Z 0.36 1923  83  27  15 0.01
Sun 08/02 21Z 0.21 1823  87  37  15 0.00
Sun 08/02 22Z 0.24 1925  88  54  15 0.01
Sun 08/02 23Z 0.24 1925  80  52  15 0.01
Mon 08/03 00Z 0.20 1926  74  49  16 0.00
Mon 08/03 01Z 0.21 1926  69  39  16 0.00
Mon 08/03 02Z 0.24 1926  68  36  16 0.00
Mon 08/03 03Z 0.26 1926  73  37  16 0.00
Mon 08/03 04Z 0.27 1926  85  60  16 0.00
Mon 08/03 05Z 0.25 1926  93  76  16 0.01
Mon 08/03 06Z 0.23 1926  96  73  16 0.02
Mon 08/03 07Z 0.25 1928  96  76  16 0.02
Mon 08/03 08Z 0.33 2029  93  86  16 0.03
Mon 08/03 09Z 0.31 2029  88  92  16 0.02
Mon 08/03 10Z 0.31 2029  83  85  16 0.01
Mon 08/03 11Z 0.34 2030  89  90  16 0.01
Mon 08/03 12Z 0.45 2028  98  98  16 0.05
Mon 08/03 13Z 0.46 2124  95  89  16 0.34
Mon 08/03 14Z 0.53 2220  92  79  15 0.24
Mon 08/03 15Z 0.64 2320  96  84  15 0.02
Mon 08/03 16Z 0.72 2420  98  81  15 0.02
Mon 08/03 17Z 0.81 2618  98  80  14 0.02
Mon 08/03 18Z 0.72 2719  97  69  14 0.09
Mon 08/03 19Z 0.63 2818  95  48  14 0.05
Mon 08/03 20Z 0.52 3018  94  31  14 0.02
Mon 08/03 21Z 0.43 3118  91  21  14 0.01
Mon 08/03 22Z 0.31 3218  84  14  13 0.00
Mon 08/03 23Z 0.28 3318  81   8  13 0.00

Run total areal avg QPF for the greens is: 1.04 inches.

Froude Number (Frd#): 
Frd# < 0.5 Flow is subcritical and blocked.
         Upslope clouds/precip backed farther upwind of and up to mtn crest.
         W Upslope snow favors eastern Champlain Valley, BTV, and Western slopes.
         Gap winds possible.
Frd# 0.5-1 Flow is subcritical/slow moving/blocked.
         Upslope clouds/precip falls immediately upwind of mtn crest.
         W Upslope snow favors western slopes to the Green Mt. spine including:
         Montgomery,Underhill,Jericho,Bolton,Richmond,Huntington,Lincoln.
         Gap winds possible.
Frd# 1-2 Flow is critical.
         With strong winds, Mountain waves/downslope winds possible.
         Precip falls close to mountain ridge crests and on lee side.
         W Upslope snow favors Green Mt Spine,Jay,Stowe,Waterbury,NE Kingdom.
Froude > 2 Flow is supercritical/unblocked(rapid flow).
         Air flows freely over terrain.
         Persistent upslope snow not favored. Scattered snow showers and flurries.

Relative Humidity (RH): 
         925-850 mb RH > 90% needed for upslope precipitation.
         with 700mb RH > 70% favors upslope snow.
         with 700mb RH > 90% greater amounts of W upslope snow possible.

850mb Temperature (T degC) Westerly Upslope Snow Ratios: (Avg=28:1)
        T > -11       10-15:1
        T -11 to -15  25-35:1
        T < -15       15-20:1