Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
287 FXUS65 KCYS 072053 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 253 PM MDT Sun Jul 7 2024 .KEY MESSAGES... - Scattered showers and thunderstorms will continue through the early evening hours, mainly in Dawes county and south of a Wheatland to Kimball line. - Confidence is growing for a potentially significant heat wave to impact the region late in the week into the first half of the weekend. && .SHORT TERM /THROUGH TUESDAY NIGHT/... Issued at 300 PM MDT Sun Jul 7 2024 Cool and somewhat unsettled weather is situated over the area today as the axis of a shortwave trough rotates through the area. Water vapor satellite imagery indicates the broad trough located over the upper Midwest today with decent mid to upper level moisture in place over our area. Current shower and thunderstorm activity is mainly focused along a 700-mb convergence boundary which has now moved south into Colorado. However, some post-frontal shower/storm activity is still ongoing. The environment is pretty marginal for storms with limited instability owing to the cool temperature. This will mainly impact areas inside a Laramie to Wheatland to Kimball triangle. Instability is a little greater in the north/eastern parts of the forecast area, so we will also be watching for some showers and thunderstorms in the Lusk to Chadron corridor. North to northwest flow is leading to some convergence in this area in the wake of the Black Hills. Shower and thunderstorm activity is not expected to last long, winding down between 00 and 03z this evening. After that, precipitation will be quite limited for a while. The messy trough will start to shift eastward on Monday as the strong ridge that has been locked in over the West coast over the last few days starts to expand eastward. Between drier air moving in and temperatures aloft climbing quickly to choke off instability, Monday is expected to be totally dry. Even afternoon cumulus will be mostly confined to the higher terrain and the Cheyenne ridge. Temperatures will remain on the cool side though as the low level cool air will be slower to evacuate. This will all produce a very nice summer day with highs in the upper 70s to mid 80s across the area. Highs will return to near normal values Tuesday as the ridge gradually strengthens. The low-level warming will eliminate the weak capping inversion, so we should see a few isolated showers/storms go up over the higher terrain, but the chance for decent rainfall will be low. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... Issued at 300 PM MDT Sun Jul 7 2024 A potentially significant heat wave is appearing more likely for a large portion of the region from Thursday through Sunday. 700- millibar temperatures are expected to soar to between 18-22 C by Friday afternoon, peaking mid-day Saturday as a highly anomalous 598-600 dam H5 ridge becomes centered over northwestern Colorado into south central and southeast Wyoming. The deterministic GFS/ ECMWF/GEM and their ensembles are in remarkable agreement w/ the strength & position of the ridge axis, especially on the days 5- 7 time scale. This is reflected quite well w/ the latest cluster analysis indicating 74% of members suggesting H7 temperatures at or above 20 deg C, above the 99th percentile & nearing the climo max per the NAEFS. The NBM spectrum is already impressive w/ the deterministic already painting a high of 97 deg F for Cheyenne & 105 deg F for Scottsbluff. This is particularly concerning given the NBM tendencies to significantly undercut highs in these very extreme scenarios. As such, have opted to go w/ the 75th %ile at this time which paints 98/105 F for CYS/BFF the MaxT on Saturday which will likely the hotter of the two days with the ridge axis directly overhead. It is worth noting that that the 95th %ile of the NBM does approach all-time record territory of 100 F For CYS w/ the latest blend, a benchmark that has not been touched since the 1950s. One reason that such values are so difficult to reach in Cheyenne is the tendency for afternoon clouds to develop once we reach the mid/upper 90s, and this could very well be the case again as the models do show appreciable vort energy aloft, along the immediate edge of the ridge. This does not seem particularly convincing at this time given the extremely dry profiles, but is a failure mode that could squash our bid at a new record & would certainly preclude a more aggressive forecast at this time. Lows are favored in the mid 60s, near 70 F in the Platte River Valley which would certainly support heat-related headlines, if current trends hold. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/... Issued at 545 AM MDT Sun Jul 7 2024 Northwest flow aloft will continue today as a secondary disturbance moves southeast across the area...resulting in another round of showers and thunderstorms this afternoon, mainly across southeast Wyoming. HAZARDS/WEATHER TRENDS: VFR conditions will prevail across southeast Wyoming and western Nebraska terminals over the next 24 hours. Showers and thunderstorms will develop in the mountains and move southeast across the I-25 corridor...mainly impacting KLAR and KCYS, and possibly KSNY this afternoon until 01z Monday. Brief MVFR CIGS and VIS are possible with this activity until conditions improve this evening. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...MN LONG TERM...CLH AVIATION...TJT