Area Forecast Discussion

KEY MESSAGES

- Isolated to scattered showers and thunderstorms this afternoon and evening with a late day cold front. A few thunderstorms may become strong to severe over eastern Montana.

- Scattered showers and thunderstorms Sunday. A few thunderstorms may become strong to severe over eastern Montana.

- Daily chance of showers and thunderstorms continue next week.

UPDATE

An area of convection (w/ a surprising amount of lightning) has been tracking WSW to ENE across northeast MT thru the night, courtesy of a weak shortwave and low level jet, and accordingly there is a wind shift to NW-N winds dropping through the NW portion of our forecast area as of 1130z. Winds are breezy at Judith Gap (gust 27mph) and have recently shifted at Billings. All models show a wind shift not making it as far east as Miles City and this will need to be watched as it will affect surface heating through the course of the day. As it looks now, a thermal trof will exist east of Billings, as pressures fall to near 990mb in north central WY. Interesting set up for today as we await the trof along the PacNW coast. Expect high temps today in the upper 70s to mid 80s over our west, while our east sees temps into the 90s, and perhaps close to 100F from Broadus to Miles City (some high res models show triple digits today, which would not be inconsistent w/ 700mb temp plume to ~15C). Daily records will be approached at Sheridan & Baker.

Heights will fall later today, which along with the arrival of a cold front and diurnal instability will yield isolated thunderstorms this afternoon and evening, beginning around 20z in our west and 00z in the east. A surge of dry air from WY will drop RHs to near 10% from Sheridan to Broadus. The dry air and very warm mid levels will largely prevent any pre-frontal initiation in this area. Again, mesoanalysis will need to be monitored today.

The day1 risk remains largely unchanged, highlighting our east with a marginal risk, and the focus of a slight risk over northeast MT and western ND, where mid levels will be cooler. All signs point to this area to our north being the favored location for severe storms today. That said, REFS shows a few updraft helicity tracks thru northern Custer Co, and to a lesser extent thru Rosebud County, on the west side of the thermal trof this evening. Strong wind gusts are a risk w/ any of today's high-based t-storms, but a bit more organization may occur in the aforementioned areas. Later in the evening, it is also possible that the development of outflow boundaries in conjunction w/ the falling heights leads to additional storm development. Any precip today will be spotty, but the chances of actual showers will increase over our far west late tonight as large scale ascent increases from the west behind the cold front.

JKL

DISCUSSION

Next 7 Days (Through next Friday)...

Warm temperatures in the 80s and 90s continue today ahead of a cold front moving through this afternoon. Scattered showers and thunderstorms (30-50% chance) will begin over the west with the frontal passage, moving northeast through the evening. The best chance for strong to severe storms is from Billings east, where southeast winds will advect moisture into the region, increasing dewpoints into the 50s F and increasing CAPE values into the 1,000-2,500 J/kg range near the Dakota border. One limiting factor for storms to develop is strong capping. If frontal forcing overcomes the capping, the main threats with severe storms will be wind gusts of 75+ mph and 1 inch hail. The Storm Prediction Center has issued a Marginal Risk (level 1 of 5) for eastern Montana and northeastern Wyoming and a Slight Risk (level 2 of 5) for the northeastern corner of Montana.

Sunday will see more widespread showers and thunderstorms (50-80% chance) as a trough moves through the region. Some strong to severe storms will again be possible over eastern Montana due to weak to moderate instability and another cold front moving through. The main threats will be wind gust of 65+ mph and 1 inch hail. If you have outdoor plans this weekend, make sure to stay weather aware and have a plan to move to a safe location if a storm approaches.

Monday may see a brief window of dry conditions under a weak ridge before the region comes under the influence of another upper level trough, bringing daily chances of showers and thunderstorms through the week. Highs will be generally in the 80s and 90s today, decreasing to 60s over the west to low 80s over the east Sunday. Temperatures will then increase to 70s and 80s by Tuesday with the weak ridge. Then, highs decrease to near to slightly below normal, in the 60s and 70s, through the end of the week. Archer

AVIATION

Issued 12Z. VFR will prevail today. Expect breezy SW winds along the western foothills including KLVM (gusts to 30kts) this afternoon, while SE winds gust near 35kts near the Dakotas border including KBHK in the afternoon/evening. A cold front arrives late in the day shifting winds to breezy NW-N winds across the region tonight, with widespread gusts on the order of 20-30kts.

The aforementioned cold front will bring isolated afternoon & evening t-storms, beginning ~20z in the west and 00z in the east. A few strong/severe storms are possible generally north and east of KBIL and up through KMLS during the late afternoon and evening. Main threat is wind gusts of 50 kts or greater. Localized/brief MVFR conditions are possible with the strongest activity. At this time, confidence of a thunderstorm impacting a TAF site is low (10-20%) so have left TS out of all 12z TAFs.

Late tonight the chance of showers increases west of KBIL. Showers may produce MVFR at times (possible impact at KLVM) while the high elevations become occasionally obscured.

JKL

PRELIMINARY POINT TEMP/POPS

Tdy Sun Mon Tue Wed Thu Fri ------------------------------------------------------- BIL 087 056/074 044/078 055/081 051/069 047/068 047/069 2/T 25/T 30/B 02/T 66/T 44/T 44/T LVM 081 048/069 035/073 046/071 040/059 037/061 037/062 2/T 45/T 20/B 05/T 98/T 55/T 55/T HDN 091 054/076 042/079 052/082 050/069 045/069 045/068 2/T 26/T 50/U 01/B 57/T 44/T 54/T MLS 096 058/078 046/077 055/084 054/070 048/069 048/068 1/B 24/T 61/U 20/B 55/T 44/T 34/W 4BQ 098 059/081 046/079 058/087 054/069 048/068 047/067 1/B 24/T 71/U 10/B 46/T 44/W 44/T BHK 095 059/082 044/075 053/086 052/070 046/066 046/066 0/N 32/T 71/U 11/B 45/T 44/T 43/W SHR 096 053/078 040/081 050/083 046/068 041/067 041/065 1/B 26/T 40/U 11/B 48/T 54/T 45/T

BYZ WATCHES / WARNINGS / ADVISORIES

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