textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Warming trend through Monday as a strong ridge of high pressure builds.

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

- Strong winds on Tuesday. A potent cold front will cross the region with valley gusts 35 to 45 mph.

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

Short Term (Through Monday Night): The period kicks off with high confidence in a gorgeous and warm weekend. With abundant 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.

While the daytime weather looks benign, a sneaky hazard will develop Sunday night. 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 gaps, with 15 to 30 mph gusts possible through Hellgate Canyon. 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 endanger anyone canoeing, kayaking, or boating on Swan Lake, McDonald Lake in Glacier National Park or portions of Flathead Lake (particularly 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): The sensible weather changes violently on Tuesday. The ridge will be shunted eastward by a deepening trough over the Pacific Northwest. A roughly 990 millibar surface low will track across southern Alberta and Saskatchewan, putting the Northern Rockies under a highly dynamic, tight pressure gradient.

Tuesday will feature a two-part wind event. Initially, a strong 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. Participants in the Mack Days fishing tournament on Flathead Lake should monitor forecasts closely.

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 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.

Highs will be tempered a bit by mid-week. Breezy afternoons will continue. A secondary shortwave may move through Wednesday, which could produce showers. Clearer skies with a cooler and drier airmass 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 80% 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

A ridge building over the region will support patchy river fog, potentially impacting KGPI or KMSO terminals this Saturday morning. Otherwise, expect VFR flying through Sunday.

MSO WATCHES/WARNINGS/ADVISORIES

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