Area Forecast Discussion
Issued by NWS Twin Cities, MN
Issued by NWS Twin Cities, MN
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086 FXUS63 KMPX 191755 AFDMPX Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 1255 PM 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 /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1245 PM CDT Thu Sep 19 2024 |Aside from a few lingering showers near (and moving away from) KEAU and a narrow swath of MVFR ceilings over central MN impacting KSTC, all sites start out as VFR this cycle. Mainly VFR conditions expected thru late afternoon, then rapidly- developing convection is expected over eastern MN late this afternoon, sliding eastward over western WI through this evening. TSRA with visibilities down to around 1SM will impact most eastern WFO MPX sites, including the potential for strong/gusty winds so have also added that mention for the afternoon-evening thunderstorms. Behind the line, quickly clearing skies with shifting winds are expected late this evening through overnight tonight. There may be sufficient moisture and calm winds at KEAU to promote ground fog close to daybreak. KMSP...Best timing for TSRA into MSP looks to be in the 22z-00z timeframe, but could last as long as 01z. Will monitor radar trends when storms develop and adjust the timing as needed. Some storms could be severe, with strong winds, large hail and a few tornadoes. /OUTLOOK FOR KMSP/ SAT...VFR. Chc SHRA/TSRA. Wind SE 5-10kts becoming W. SUN...VFR. Slight chc SHRA/MVFR. Wind NW 5-10 kts. MON...VFR. Slight chc SHRA/MVFR. Wind NE 5-10 kts. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...None. WI...None. && $$ DISCUSSION...NDC AVIATION...JPC