textproduct: Great Falls

KEY MESSAGES

- Showers and a few thunderstorms this afternoon, mostly across Southwest Montana.

- Spring system brings a mix of rain and snow to the region from late tonight through Friday morning.

- Dry and mild this weekend into early next week.

DISCUSSION

- Meteorological Overview:

Flow aloft across the region will become increasingly southwesterly through the day today ahead of a robust upper low moving onshore across the Pacific Northwest this evening. The main impact today will be for a mix of rain and snow showers, mostly near terrain in Southwest and Central Montana from this afternoon into the evening.

As the core of the upper low shifts across the region into Thursday morning, strong forcing (With some convective enhancement at times) will result in a mix of rain and snow initially, with a transition to nearly all snow heading into Thursday evening as snow levels fall with the arrival of cooler air. Heavy mountain snow is most favored from the Little Belts and the Gallatins/Madisons and adjacent ranges through Friday morning before precipitation winds down.

Upper level ridging then builds in, more or less persisting through Tuesday. Weak embedded disturbances rippling through the ridge will result in brief periods of precipitation, but those instances look very light at this time. -AM

- Forecast Confidence & Scenarios:

Showers and thunderstorms today:

HREF probability for surface based CAPE in excess of 100 m2/s2 increases above 50% across areas southwest of a Helena to Bozeman line by 3 PM this afternoon. Combine this with at least weak initial forcing ahead of the main upper low approaching, and numerous showers (Lower elevations) and snow showers (Higher elevations) should develop as a result. At least a few instances of lightning also appears to be reasonable from mid-afternoon through early evening as well.

This convective activity shifts northeastward before weakening this evening. Additional convective activity (Largely elevated) pivots across Southwest Montana late tonight as the core of the upper low continues its approach. Given the timing, snow showers at lower elevations will be more common late tonight into Thursday morning.

Convective activity Thursday, including potential for snow squalls:

Potential for additional convective activity into late Thursday morning and the afternoon will largely depend on how quickly surface low pressure across Southwest Montana or Northwest Wyoming tracks eastward and draws cooler air southward. A slower to progress surface low would favor additional instability and associated showery activity, while a quicker surface low would favor less convective activity (But higher snow amounts behind the front). Should the slower solution verify, the risk for intense snow showers and perhaps a snow squall, would increase along the cold front as it progresses across Southwest Montana Thursday afternoon.

Mountain snowfall Thursday into Friday:

Orographic forcing combined with favorable dynamics and some convective enhancement at times will result in periods of heavy mountain snow, mostly in the Little Belts/adjacent areas and the Gallatins/Madisons/adjacent areas. The probability for 9 inches of snow through Friday afternoon is at or greater than 50% at Kings Hill, Targhee, and Raynolds Passes.

Lower elevation snowfall Thursday into Friday:

There continues to be quite a lot of sensitivity with respect to snow amounts at lower elevations, largely tied to just how quickly rain transitions to snow at lower elevations. Even if that transition occurs during the day Thursday, at least some melting will eat into snow totals at lower elevations in many areas. Probabilities for impactful snow at lower elevations (Think 6" or so) have largely decreased over the past 24 hours. Areas that are still favored for at least a few inches of snow are in northwesterly upslope areas across Central Montana. Additional areas will see a few inches of snow at lower elevations, but confidence in these locations is not yet high. -AM

AVIATION

01/06Z TAF Period

Low clouds some areas of light precipitation across southwest MT will maintain mountain obscuration with MVFR to occasional IFR conditions at KEKS and KBZN, mainly for low ceilings. VFR conditions generally prevail at central/north-central MT terminals with some risk for low cloud/fog development at KCTB (30-40%) and to a much lesser degree (10%) at KGTF. Showers increase from west to east across the area late Wednesday afternoon. Hoenisch

PRELIMINARY POINT TEMPS/POPS

GTF 26 56 35 43 / 10 10 70 100 CTB 19 51 30 38 / 0 0 50 90 HLN 30 56 34 44 / 30 50 70 100 BZN 28 55 33 48 / 70 50 60 100 WYS 31 48 30 40 / 80 70 90 100 DLN 33 57 35 44 / 50 60 70 90 HVR 21 52 29 42 / 0 0 40 90 LWT 21 50 29 46 / 20 10 40 90

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from 6 AM Thursday to 6 AM MDT Friday for East Glacier Park Region-Elkhorn and Boulder Mountains- Northwest Beaverhead County-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Storm Warning from 9 AM Thursday to noon MDT Friday for Gallatin and Madison County Mountains and Centennial Mountains- Little Belt and Highwood Mountains.

Winter Weather Advisory from 3 PM Thursday to noon MDT Friday for Cascade County below 5000ft-Fergus County below 4500ft- Judith Basin County and Judith Gap-Snowy and Judith Mountains.

Winter Weather Advisory from 9 AM Thursday to noon MDT Friday for Big Belt, Bridger and Castle Mountains.