Area Forecast Discussion

KEY MESSAGES

- Late day isolated to scattered showers and thunderstorms. A few thunderstorms may become strong to severe over east central Montana (think Musselshell to northern Custer County).

- Scattered showers and thunderstorms Sunday. Severe thunderstorms are possible over southeastern Montana and north central Wyoming.

- Mainly dry Monday; then a daily chance of showers and thunderstorms rest of work week.

DISCUSSION

Next 7 Days (Through Friday)...

Satellite imagery showed an upper low over the Pacific NW with southwest flow over our region. There were indications of energy ejecting from the bottom of the parent low and tracking into western Montana and northwest Wyoming. These perturbations are expected to trigger convection through the afternoon over south central Montana which will move northeast into a moisture rich environment late today. MUCAPE values from 500-1250 J/kg are progged by mid afternoon over much of our CWA, highest toward Rosebud/Custer Counties. Mean effective shear will range from 25 to 45 kts while modest updraft helicity is progged from Wheatland to Musselshell, with higher values ramping up this evening from northern Custer into northeast Montana. Soundings lean heavily toward inverted V's. This suggest an isolated wind threat (60+ mph) and a lesser threat of any hail for our CWA. Large hail looks much better with significant helicity into NE MT this evening pushing into North Dakota toward Williston. SPC currently has marginal severe area over portions of our central and eastern CWA for late today which looks reasonable.

A cooler air mass with northerly winds will drop into our CWA this evening, then push all the way through to our southeast sections by Sunday morning as the parent upper low over the PAC NW lifts through western MT into southern Saskatchewan Sunday. An axis of instability from the western Dakotas into southeast MT is expected with decent forcing from the cold front and associated jet energy aloft. Strong deep layer shear should lead to a large hail/damaging wind threat over eastern portions of our CWA and possibly into Sheridan County, WY. Latest helicity proggs are pegging Powder River and Carter Counties. How far west this threat is depends on how much the cooler air mass gets into our area limiting instability. At any rate, our SE MT section is under SPC slight risk to enhanced for severe weather Sunday afternoon and early evening. This may be situation which is in high flux as many variables will be in play. Something to closely monitor for those with plans Sunday.

Much drier air moves in behind the weekend system for Monday as a weak ridge develops. For mid week we come under the influence of another upper level trough developing over the Rockies. This will bring a daily chance of showers and thunderstorms through the rest of the work week...mostly garden variety with little chance of severe storms.

After highs in the 80s and 90s today, look for cooling to 60s over the west to low 80s over the east tomorrow. Temperatures will then increase to 70s and 80s Monday and Tuesday before cooling into the upper 60s to mid 70s for rest of week. BT

.AVIATION...

18z Discussion...

VFR conditions will prevail for most of the period. Winds out of the ESE will flip to N behind a FROPA which should move through KBIL around 00z and KSHR around 01z. Gusts may reach up to 20 kts with the FROPA.

The main concern for the period is for strong to severe storms between 22z and 04z. CIGs and VIS in and near the storms will remain VFR but the main concern is winds. Conditions are favorable for the development of dry microbursts under and near convection as well as outflow boundaries extending well out away from any storm. Strong erratic winds may locally gust up to 60 kts near KMLS and 50 kts near KBIL and KSHR. Winds in the TAFs we kept slightly lower due to the higher confidence in those speeds. All convection should be out of the forecast area by 05z. WMR

PRELIMINARY POINT TEMP/POPS

Sun Mon Tue Wed Thu Fri Sat ----------------------------------------------------------- BIL 057/075 044/080 055/079 050/068 047/070 048/070 048/072 24/T 20/B 03/T 75/T 34/T 22/T 22/T LVM 048/068 035/075 047/070 039/059 036/063 039/066 039/066 34/T 10/B 08/T 97/T 43/T 23/T 33/T HDN 054/077 041/081 053/082 048/068 045/069 046/070 046/071 24/T 20/B 02/T 76/T 43/T 22/T 22/T MLS 057/079 045/078 057/084 053/068 049/067 049/069 048/070 35/T 40/U 22/T 55/T 33/T 22/T 12/W 4BQ 059/082 046/081 059/087 051/066 047/065 048/068 048/069 25/T 50/U 20/B 77/T 33/T 22/T 22/T BHK 059/083 044/077 054/087 051/066 046/065 046/067 045/067 34/T 50/U 21/B 46/T 34/T 22/T 22/W SHR 053/080 039/083 050/082 045/065 040/066 041/067 042/069 25/T 40/U 12/T 78/T 43/T 23/T 33/T

BYZ WATCHES / WARNINGS / ADVISORIES

MT...None. WY...None.