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Area Forecast Discussion
Issued by NWS Twin Cities, MN
Issued by NWS Twin Cities, MN
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697 FXUS63 KMPX 140550 AFDMPX Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 1250 AM CDT Wed Aug 14 2024 .KEY MESSAGES... - Our dry period ends tomorrow with widespread showers expected to gradually spread west to east tomorrow. The heaviest rain is expected tomorrow evening through early Thursday morning. - Lingering showers are possible Friday & Saturday before we resume our dry and mild stretch by Sunday. && .DISCUSSION... Issued at 252 PM CDT Tue Aug 13 2024 A broad cumulus field has blossomed over the northern plains this afternoon with cumulus evident from southern Canada all the way south through Illinois, Indiana, and Ohio including most of Minnesota & Wisconsin. There are some high clouds moving through southwestern Minnesota as well evident on GOES satellite imagery, with a small chance for a few pop showers much like yesterday in southwestern Minnesota. Light southerly to southeasterly winds continue ahead of the incoming system which will bring showers back to the region tomorrow, alongside a northwesterly wind shift that looks to persist through the next several days. The impetus for initial showers arriving in western Minnesota as early as sunrise tomorrow is a strengthening low level jet bringing an influx of moisture and warmth to force showers as the leading edge of a trough begins to slide eastwards over the northern CONUS Rockies. This jet will continue to pump gulf stream moisture northwards allowing for increasing PWATS into the 1.7-1.9 range which is already a couple tenths of an inch above the 90th percentile for August 14th. A series of shortwave impulses are expected to spread eastwards from the approaching trough which should couple with the jet forcing to produce a large swath of showers and potential storms which will gradually move west to east across Minnesota and into Wisconsin tomorrow. Model guidance has a bit of spread in regards to timing and amount of precipitation with the CAMs favoring a slower onset time and less overall precipitation compared to the medium to long range global models, with significant spread in the GEFS in regards to both total amount of rainfall and the timing. The GEFS mean for MSP would produce an inch and a half of rainfall through 7pm Thursday with a lack of consensus within guidance as most membership spreads from a few tenths to as much as 2 inches. The higher amounts would be possible during the heaviest rainfall which looks to be late tomorrow through around 5-6am Thursday as the low level jet once again strengthens and amplifies the forcing from the trough. The surface response is fairly muted with a surface low only forming as the trough axis swings through by midday Thursday. Overall, expect most of the area to pick up anywhere from 3/4 of an inch to around an inch and a half of rain over a 36 hour period from when precipitation enters the region tomorrow through Thursday. Taking a deeper dive into severe weather parameters, there is also quite a bit of spread in potential as the higher pwats and fairly saturated moisture profile would support heavy rain but only weak storms, especially overnight. As CAM guidance has trended slower than global guidance, the best chance for stronger storms would be near sunrise Thursday with a strong low level jet present alongside increasing instability. As of right now, the main threat would be heavy rain producing outflow or micro-burst type winds, with that threat looking on the lower end of things for now. As the upper level system rotates over the Great Lakes heading into the weekend, further light showers are possible as both aloft and surface winds shift northwesterly, with cold air advection showers possible until the system rotates out further by Sunday. Once we get through Saturday, the rest of the forecast looks dry and a return to what we have been seeing the last week or so, which is mild temperatures and dry weather. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1242 AM CDT Wed Aug 14 2024 Initialized with VFR and light winds tonight, but patchy fog has already developed in western WI (specifically EAU). Fog may be possible at other sites outside of MSP but confidence not high enough to include at other sites at this point. Will continue to monitor through the early morning hours for possible TAF updates. The main focus of these TAFs remains the rain for Wednesday evening through Thursday. Rain will develop from southwest MN and progress eastward. As rain starts, a drop to MVFR is expected and eventually some IFR is expected as well. The coverage of IFR is still uncertain, but confidence has increased such that IFR ceilings were added for western and central MN sites. Rain chances will continue past the end of the current TAFs with degraded conditions likely continuing into Thursday morning. KMSP...Initialized with VFR conditions and light winds which will persist through early Wednesday afternoon. Conditions then deteriorate late Wednesday afternoon onward as the rain shield moves into eastern MN. Still a little bit of wiggle room as to the start time so have maintained the PROB30 mention for a potential of rainfall to move in prior to 00z but confidence increases significantly from 00z onward that rain will move across the MSP airspace. Low confidence on CB/TS so have omitted such mention at this point. Regardless, MVFR conditions at best will persist through Wednesday night, possibly dropping to IFR by Thursday morning. Will see how the model guidance develops such a scenario for later TAFs. /OUTLOOK FOR KMSP/ FRI...VFR, chc MVFR/-SHRA. Wind NW 10-15 kts. SAT...VFR. Wind NW 10 kts. SUN...VFR. Wind NW 5-10 kts. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...None. WI...None. && $$ DISCUSSION...TDH AVIATION...JPC