textproduct: Great Falls

DISCUSSION

- Meteorological Overview:

Through Tuesday night:

Upper level ridging is in its initial stage of building in across the western US this afternoon. A lingering shortwave quickly exiting eastward ahead of the building ridge is resulting in cumulus fields so far this afternoon across the Hi-Line and near the Idaho border. Latest RAP mesoanalysis and forecast shows a few hundred J/Kg instability in these areas through the afternoon and early evening. Isolated showers and perhaps a couple thunderstorms seems reasonable through the remainder of the afternoon. Given the already synoptically gusty conditions across North-central Montana, and forecast soundings near West Yellowstone featuring modestly deep inverted-V profiles, gusty winds would be the primary concern with any thunderstorms in both of these areas through the evening.

Ridging continues Tuesday and Tuesday night, allowing for a largely benign day across the region. A few high resolution guidance members develop a few terrain based showers across SW MT Tuesday afternoon, but they seem to be the outlier at this time.

Wednesday into Thursday:

An active stretch of weather begins Wednesday. A potent upper level low cuts through the upper ridging through the day Wednesday across the Pacific Northwest and eventually the Northern Rockies. Ahead of this upper low, southerly to southwesterly winds develop across the region, which will result in warm air advection across the region. Temperatures look to peak across the forecast area Wednesday, mainly in the upper 80s or lower 90s. While the hot temperatures are certainly notable, as they look to be approaching record levels for the date, the primary concern Wednesday will be for the combination of dry and gusty conditions resulting in some localized fire weather concerns (See Fire Weather Section for more details) in addition to scattered showers and thunderstorms bringing potential for strong and erratic winds gusts.

Concern then transitions to strong westerly flow aloft developing along side and behind the passage of the upper level low Wednesday evening and overnight. This flow begins to be translated to the surface near the Rocky Mountain Front Wednesday night before spreading elsewhere through the day Thursday as daytime mixing ensues. There has been quite a bit of run to run model guidance volatility on the overall strength of winds for Thursday, though it should be noted that the latest trend has been toward stronger winds.

Snow levels look to fall sufficiently low enough to result in some mountain snow by Thursday morning. While some light pass level snowfall cannot be ruled out, it would be difficult to see a scenario where this snow would be able to accumulate on any roads given warm temperatures preceding the snow.

Friday into the weekend:

An additional quick moving shortwave moves in from the Eastern Pacific Ocean and Pacific Northwest Friday. Details at this range for this system are murky, but another stretch of breezy to gusty winds along with another opportunity for precipitation will accompany this system Friday into Saturday.

Active weather looks to persist into at least Sunday, evident by cluster guidance showing high confidence for troughing across the west. That said, individual cluster members paint different pictures as to timing and strength of troughing. The main takeaway will be for cooler temperatures and opportunities for precipitation. -AM

- Forecast Confidence & Scenarios:

Wednesday Thunderstorm Risk:

Wednesday will be an intriguing day for several reasons, but I will keep this discussion to thunderstorm potential.

A belt of stronger southwesterly winds aloft looks to arrive through the day Wednesday ahead of the upper level low moving east from the Pacific Northwest. Daytime mixing will result in these stronger winds mixing to the surface, resulting in a synoptically gusty (Perhaps strong gusts in SW MT) day across much of the region. Forcing for ascent will overspread the region in the afternoon as the upper low continues its approach. Sufficient instability looks to be in place to result in scattered showers and thunderstorms during the afternoon and evening. The already synoptically gusty conditions along with very deep inverted-V soundings will result in the risk for isolated pockets of very strong outflow gusts in areas where the strongest thunderstorms form.

Mitigating factors in this setup that would ultimately hinder the risk for strong thunderstorm gusts would be ample cloud cover limiting daytime heating. This would limit overall coverage of thunderstorms, resulting in a more isolated risk for stronger outflow gusts.

Synoptic winds across SW MT Wednesday:

As mentioned prior, a belt of stronger SW flow aloft overspreads the region during the day Wednesday. Guidance is becoming more consistent with this signal for stronger winds. That said, I'd like to see another model cycle or two of consistency before considering a High Wind Watch for these areas.

Winds Wednesday Night into Thursday:

Confidence is increasing in strong westerly mid-level flow developing on the backside of the upper low progressing east of the region Wednesday night into Thursday. That said, there is still a small piece of uncertainty with respect to timing the strongest winds aloft. Presently most guidance moves these strongest winds across the region late Wednesday night, during a timeframe that would not be conducive to widespread gusts being realized at the surface. Should the timing of the stronger winds aloft be delayed by 6 hours or so, concern would rise for these much stronger winds mixing to the surface Thursday across much of the region. Should this scenario unfold, impacts from the gusty winds would be higher compared to a similar magnitude event during winter, simply from the perspective that trees are developing leaves.

This timeframe will continue to be monitored for trends in timing and confidence levels. -AM

AVIATION

12/00Z TAF Period

VFR conditions and generally light winds will prevail through the TAF period with only a few passing high clouds expected through the day on Tuesday. The exception will be along the Rocky Mountain Front where winds will pick up Tuesday afternoon with gusts up to 35kts possible through the evening hours. -thor

FIRE WEATHER

Elevated to near-critical fire weather conditions are expected on Wednesday as temperatures soar well above normal and into the mid- 80s to low 90s, with the potential for numerous locations to set new daily record highs for May 13th across Southwest through North Central Montana. The warm temperatures will be accompanied by strong and gusty south to west winds; most notably along and south of the I- 90 corridor within the north-south orientated valleys of Southwest Montana, west of the I-15 corridor over the plains of Central and North Central Montana, and across Hill and Blaine Counties. Further enhancement of winds are likely beneath and/or near shower and thunderstorm activity, especially across Southwest and portions of Central Montana or generally along and south of the US Hwy 12 corridor. Dry conditions, with relative humidity values in the teens will be common across much of the plains and valleys. Probabilities for the Hot-Dry-Windy Index (HDWI) rising to within the 95Pct of Climatology has increased to a 60% or greater chance across most lower elevations across Southwest Montana south of the I-90 and across Hill and Blaine Counties in North Central Montana, with Fire Weather partners indicating that fuels would be receptive to fire, especially across Southwest Montana. For this reason a Fire Weather Watch has been issued for much of Southwest Montana and Fire Weather Zone 113 in North Central Montana from 18z Wednesday through 03z Thursday. - Moldan

..RECORD OR NEAR RECORD MAXIMUM TEMPERATURES FORECAST FOR MAY 13

LOCATION FORECAST HIGH RECORD HIGH Boulder 84 83 Bozeman MSU 89 86 Chester 89 86 Choteau 89 85 Conrad 89 87 Cut Bank 86 84 Dillon 88 87 Fort Benton 94 88 Great Falls 91 89 Helena 90 89 Havre 93 92 Lewistown 88 88 Shelby 87 84 Stanford 88 86 West Yellowstone 83 81

PRELIMINARY POINT TEMPS/POPS

GTF 43 79 50 89 / 0 0 0 30 CTB 38 76 46 82 / 20 0 0 30 HLN 42 83 48 89 / 0 0 0 30 BZN 39 81 45 88 / 0 0 0 20 WYS 36 80 41 81 / 20 0 0 20 DLN 42 83 49 84 / 0 0 0 20 HVR 38 78 47 92 / 20 0 0 20 LWT 39 74 46 88 / 0 0 0 20

TFX WATCHES/WARNINGS/ADVISORIES

Fire Weather Watch from Wednesday afternoon through Wednesday evening for Hill and Blaine Counties.