textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Potentially strong thunderstorms each afternoon/evening today through Wednesday

- Primary threats from thunderstorms will be strong, gusty winds. For tonight, there will be about a 10-15% chance of thunderstorm gusts exceeding 50 mph

- A cool and wet pattern arrives Tuesday, lowering temperatures and bringing widespread rain through Friday.

Satellite imagery this afternoon is depicting an elongated low pressure circulation approaching the coast of the Pacific Northwest. This feature will be a key player in the weather over the next several days. In the near term, flow aloft ahead of this circulation over the Northern Rockies will be southwesterly this afternoon, turning southerly tonight as the low moves south and east. South and southwesterly flow are effective in transporting warmer air and Pacific moisture, key ingredients for convective storms, into the Northern Rockies. Clear skies ahead of the warm, moist air will aid in low-level heating necessary to kick off convection later in the day. Model soundings indicate good instability and a dry lower atmosphere, indicating a primary threat of strong outflow winds.

High resolution models indicate showers and thunderstorms developing over Idaho, Lemhi, and southern Ravalli counties by about 2 pm MDT. Storms track to the north or northeast, reaching west-central and southwest Montana (including Hamilton, Missoula, Butte, and I-90 east of about Missoula) by about 3 pm, and lasting until about 8 pm before continuing north through Lake, Powell, and Flathead counties including Flathead Lake. Storms will produce strong, gusty winds of about 40 mph with about a 10-15% chance of exceeding 50 mph. Small hail and frequent lightning will accompany these storms. Tonight, a cold front associated with the main low pressure circulation will enhance chances for overnight convection from about 8 pm to midnight MDT. This second round of storms will be focused further west, including north-central Idaho and northwest Montana.

The low becomes positioned to the southwest of the Northern Rockies on Tuesday. Expect another round of afternoon convection, along with more stratiform rain through Wednesday. Most model guidance keeps the low over Nevada into later this week before ejecting northeast this weekend. That scenario will keep a southwesterly moisture feed available in the Northern Rockies, and unsettled weather will accompany that moisture feed.

AVIATION

Radar returns and lightning detection systems are already showing convection developing over southern Lemhi County early this afternoon. Moisture and warm air from the southwest will be conducive to widespread convective storms this afternoon and tonight. Model soundings indicate ample instability and shear with a dry lower atmosphere. The resulting storms will pose a threat of strong, erratic outflow winds, small hail, brief heavy rain decreasing visibility, and frequent lightning. Expect thunderstorm gusts of about 40 knots, with about a 10% chance of exceeding 50 knots. Convective-allowing models indicate storm development will take place from south to north, starting in Lemhi and Idaho counties as early as 25/2000Z, moving north to Hamilton, Missoula, and Butte by about 25/2100Z, Kalispell by about 25/2200Z. A second round of convection will arrive overnight as a cold front will enhance instability (26/0200Z to 26/0600Z). Another round of thunderstorms are in the forecast for Tuesday afternoon/evening.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.