Area Forecast Discussion
Issued by NWS Twin Cities, MN
Issued by NWS Twin Cities, MN
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899 FXUS63 KMPX 191140 AFDMPX Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 640 AM CDT Thu Sep 19 2024 .KEY MESSAGES... - Potential for very large hail today with a Slight Risk (level 2 of 5) this afternoon and evening. - Rain is increasingly likely this weekend, but uncertainty remains high with the rainfall forecast. - Our summer like warm with highs in the 80s continues through Saturday before cooler, more early fall like, air arrives Sunday with highs retreating back into the 60s and 70s. && .DISCUSSION... Issued at 258 AM CDT Thu Sep 19 2024 One look at any satellite image of North America and you will easily be able to see the main low driving our weather. It is currently spinning near the Montana/North Dakota/Saskatchewan border area. This low will continue to occlude farther as it slowly tracks across the Canadian Prairies. Closer to home, a cold front will track across Minnesota today. This will help provide the forcing needed for our storm chances later today. Early this morning we have seen more or less isolated air mass thunderstorms with little risk of anything strong. This will continue to be isolated in coverage through the morning with little severe chance thanks to weak instability at this time of the morning. This afternoon into evening will be the main event. Mid level lapse rates will deepen through the day today setting up a favorable thermal profile. Surface heating should overcome any capping later today allowing for CI. Forecast CAPE values in the CAMS is generally in the 2000 to 3000 J/kg range. With this instability and the shear from veering winds we should have an environment supportive of supercells. The question is where will the forcing be best once the CI starts to occur? Current guidance favors east central and south central Minnesota. This is from the forecast position of the front once enough surface heating has occurred to lead to CI. Generally along and to the east of the I-35 corridor still seems like the best chance. With ample CAPE and high values of SHIP, there are numerous large hail analogs in forecast soundings. So hail will be the main threat, but with supercells all severe threats remain possible. Drier weather moves in on Friday with high pressure moving in from the central plains behind the frontal passage. Despite a cold frontal passage WAA should keep Friday temperatures well above normal in the 80s. This WAA will also be in place Saturday making it another above normal temperature day. The frontal passage Saturday into Saturday night should finally cool us down with northerly winds aloft and CAA. This should provide for a strong enough cold front to give us a decent chance at rain on Saturday. Looking towards ensemble guidance, most members agree and have rain on Saturday. However the rainfall amounts still have a significant amount of spread, so the uncertainty in the forecast remains high. So while most will see rain, it could be anything from a few hundredths to a good soaking rain of 1-1.5 inches. This should start to get better resolved in guidance once we get past our current system and the wave driving this round moves into western North America. Sunday into next week continues to have more chances for rain, but there is much more spread within ensemble members tied to multiple shortwaves transiting the north central CONUS. What remains likely is cooler, but closer to normal temperatures next week. && .AVIATION /12Z TAFS THROUGH 12Z FRIDAY/... Issued at 640 AM CDT Thu Sep 19 2024 Isolated thunderstorms will continue this morning mainly impacting the Wisconsin terminals. Another round of storms are expected this afternoon into evening. Best chances at MSP, MKT, RNH, and EAU with a chance at STC. This afternoon into evening`s round will be the main event with MVFR to IFR expected as the storms move through. Should be more around an hour in impacts with the timing of TEMPO and prevailing groups longer in the 12Z TAFs due to uncertainty. This will be revised in future issuances of the TAFs as we get closer and confidence increases. Outside of the storm chances just a shift from southerly winds to westerly winds as a front moves through today. KMSP... We should be done for the rest of the morning now at the terminal for storms. The next round will be between 21Z and 00Z. The actual time for storms should be more around an hour, but uncertainty in timing has kept the window larger. /OUTLOOK FOR KMSP/ FRI PM...VFR. Wind SE 5-10kts. SAT...VFR. Chc SHRA/TSRA. Wind SE 5-10kts. SUN...VFR. Chc SHRA/MVFR. Wind NE 5-10 kts. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...None. WI...None. && $$ DISCUSSION...NDC AVIATION...NDC