textproduct: Great Falls

KEY MESSAGES

- A cold front pushes through the area today, bringing showers and thunderstorms, some of which may bring some heavy rainfall across Southwestern Montana. - Closer to average to start next week, but afternoon showers and thunderstorms will remain common. - Potential warm spell later next week.

DISCUSSION

- Meteorological Overview:

A cold front will push through the area today, bringing with it a round of showers and thunderstorms as it pushes through the area. Current radar and satellite are showing some thunderstorms beginning to develop across Southwestern Montana early this afternoon, and these will continue to develop and move northeastward through the evening. While the forcing from the cold front will help to organize storms a bit more than recent days, very weak shear will keep things from getting too strong, though higher than average PWATs (precipitable water) and tall, skinny atmospheric profiles suggest that some of the storms could be on the heavy side and be efficient rainfall makers. While our soils should be able to handle a burst of heavy rain for the most part, the big concern is that a heavier thunderstorm moving directly over a burn scar (such as the Horse Gulch) could result in some flooding. While we are not expecting this to be an issue since the best potential for such rainfall is further south, we will keep a close eye on things today.

As we head into next week, northwesterly flow aloft will keep our temperatures near average with a few pop up afternoon showers and thunderstorms, particularly Sunday and Monday. Upper level ridging will nudge back into the area Tuesday and Wednesday as the summertime ridge begins to build into the Western US, though there will still be a few opportunities for weak systems to ride on the north side of the ridge and bring us some more showers and storms through the second half of the week. Even with these systems, temperatures look to begin to nudge warmer, with above average temperatures possible Thursday and Friday.

- Forecast Confidence & Scenarios:

Saturday Thunderstorms: A cold front moving through the northwestern US on Saturday will bring a round of showers and thunderstorms to the area as it pushes through. While a widespread severe outbreak is not expected at this time, the presence of a forcing mechanism (the front), modest instability, and modest shear profiles indicate there is some potential for the storms to be a bit better organized than storms we have had recently. Given this, I cant rule out the possibility of a stronger storm or two that would be able to produce some sub-severe wind or hail (58 mph gusts or 1 inch diameter hail) ahead of the front. The larger concern, however, will be the possibility of some heavier, torrential downpours with a few of the thunderstorms. Burn scars, such as the Horse Gulch, will be the main concern, but some ponding in low lying areas is also possible.

Warmth next week: There seems to be decent agreement in the synoptic models that our upper level ridge will become notably stronger than average late next week. Looking at the WPC Cluster Analysis tool, we can see that, at least for Thursday, around 80% of the ensemble members (this tool looks at the Canadian, GFS, and ECMWF ensemble members) are showing above average 500 mb heights, which generally lead to warmer than average temperatures under the higher heights. An approaching upper level low will likely cool things off by the end of the week, with nearly 65% of the ensemble space showing a cooldown by Saturday. Ludwig

AVIATION

20/12Z TAF Period

Scattered showers/thunderstorms will affect Southwest MT through about 12z Sun. Isolated thunderstorms will develop over North Central MT, mainly near the US/Canadian border after 00z. Mountains/passes will be obscured at times by passing showers/thunderstorms. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 52 73 48 66 / 40 20 20 10 CTB 46 67 43 62 / 30 60 50 10 HLN 52 79 49 71 / 20 0 0 20 BZN 47 78 45 73 / 50 0 10 10 WYS 39 69 37 71 / 90 30 20 0 DLN 46 77 45 74 / 70 0 0 0 HVR 50 73 45 66 / 60 0 20 30 LWT 48 70 43 62 / 40 10 40 50

TFX WATCHES/WARNINGS/ADVISORIES

None.