textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Warming trend through Monday under high pressure.

- Sunday Evening/Night: Breezy east winds along and west of the Divide. This poses slight risk to small craft on Swan Lake, McDonald Lake and Yellow Bay of Flathead Lake.

- Strong winds on Tuesday. A potent cold front will cross the region with valley gusts 35 to 45 mph, reaching 50 mph over higher terrain and along the divide.

- Widespread freezing morning lows Wednesday through Saturday of next week.

Short-Term (Through Monday Night): Satellite imagery this afternoon shows abundant sunshine as high pressure has strengthen over the region. With this sunshine, we will easily mix down warmer air, allowing afternoon highs to soar into the 60s and low 70s across the western Montana and central Idaho valleys by Monday.

Those planning outdoor recreation activities Sunday late afternoon and evening should prepare for a north-northeast wind shift. As a polar airmass plunges east of the Divide, an easterly pressure gradient will set up over western Montana. High- resolution guidance (including the GFS-WRF and REFS) shows breezy easterly winds pushing through the terrain gaps, with 15 to 30 mph gusts. These types of easterly winds can have varying strengths depending on what kind of gradient develops. If the gradient is stronger, then the easterly winds can pose hazards to anyone canoeing, kayaking, or boating, particularly on Swan Lake, McDonald Lake in Glacier National Park or portions of Flathead Lake (near Yellow Bay). Water temperatures are currently hovering around a dangerous 40 degrees. These evening gap winds can develop quickly and catch recreationists off guard, posing a severe cold- water shock and drowning hazard.

Mid-Term (Tuesday through Thursday): Ensemble model guidance is in strong agreement (>90% of members) for the ridge to breakdown quickly on Tuesday, with a cold front bringing widespread gusty to locally strong winds. A roughly 990 millibar surface low will track across southern Alberta and Saskatchewan, putting the Northern Rockies under a dynamic, tight pressure gradient.

Tuesday will feature a two-part wind event. Initially, a southerly gradient will develop Monday night. Low temperatures will be unseasonably mild in some places. Winds could already be gusting over 20 mph at Flathead Lake a little by or after daybreak.

By Tuesday morning into the afternoon, the cold front sweeps through, driven by moderately strong pressure rises. West to southwest winds will increase, with high probability (>80 percent) for wind gusts 25 to 35 mph across the region. There are a few areas that could experience gusts greater than 45 mph along with isolated wind damage: 90% Ashley Lake area and Marias Pass, 80% Anaconda, MacDonald Pass and Marias Pass, 65% central Bitterroot Valley, US-2 west of Kalispell, and 55% at Lakeside.

Temperatures will trend lower by mid-week behind the front, with breezy winds each afternoon. A secondary shortwave may move through Wednesday, which could produce showers. Clearer skies with a cooler and drier air mass will allow radiational cooling to drop morning lows into the 20s for the remainder of the week.

Long-Term (Friday into Next Weekend): Transient ridging looks to build back in briefly by Friday and Saturday, yielding another quick warm-up. However, ensemble cluster analysis strongly suggests this relief will be short- lived. Over 70% of the ensemble space favors long-wave troughing digging back into the Pacific Northwest or Great Basin by Sunday, April 12th. While the exact depth and moisture availability of this next system remains uncertain, persistence and the overall pattern strongly favor a return to unsettled weather to close out the weekend.

AVIATION

Skies have generally cleared across the Northern Rockies airspace under high pressure. Westerly gusts of 10-20kt will develop after 04/2000Z, primarily along the divide and over the higher terrain. Little to any fog development is expected tonight as dry air settles into the region today. Mid to high cloud cover will increase west to east Sunday morning.

MSO WATCHES/WARNINGS/ADVISORIES

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