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FXUS63 KFGF 201154 AFDFGF

Area Forecast Discussion National Weather Service Grand Forks ND 654 AM CDT Mon May 20 2024

KEY MESSAGES

- The probability for greater than 1 inch of rainfall is 60% Tuesday into Wednesday across parts of northwest and west central Minnesota.

UPDATE
Issued at 654 AM CDT Mon May 20 2024

Patchy shallow/ground fog has develops in areas of clearing across parts of the southern Red River Valley and a few locations have reported visibility drops to 1/4 mile, though webcams show limited impacts due to the very shallow nature of most of this fog. I made some adjustments to add patchy fog to the morning period and issued a Special Weather Statement through 8am when daytime mixing should allow this fog to burn off or lift.



DISCUSSION
Issued at 347 AM CDT Mon May 20 2024

Southwest flow remains in place with mid/upper troughing to the west centered across the US Norther Rockies towards the Great Basin. As this trough shifts east, a more organized/stronger mid/upper low develops into the Northern Plains/Upper Midwest to our southeast and brings a widespread rain event to parts of our region. Following this the pattern becoming more disorganized with some variation of either zonal as a series of additional waves are resolved to move west to east through an active northern jet stream. There is higher variation in timing/evolution of these features but there is a growing consensus on another period of organized rain/embedded thunderstorms Thursday night into Friday. Temperature trends vary from seasonal in the 60s/near 70 (like today) to below normal particularly on days like Tuesday when rainfall/cloud cover could limit daytime heating (50s for highs where rain occurs).

Tuesday-Wednesday Rain:

Guidance continues to show general consensus of a "Colorado-low" type pattern Tuesday into Wednesday with an organized deformation rain band forming a swath of moderate to heavy rain primarily in west central/northwest MN. Subtle shifts/differences in strength/position of the mid level wave and interaction/position of a stalled frontal zone could shift this axis a bit farther west like some guidance shows, however a growing consensus of ensemble data continues to favor our east/southeast for highest precip chances/amounts and our west-northwest (Devils Lake Basin) may remain mostly dry for the event. Duration of rainfall lowers any risk of excessive runoff, so the main impacts will be to outdoor plans/agriculture planting.
Probability for 0.5"+ for the event now ranges from near 60% around Fargo to over 80% in west central MN (near zero in Devils Lake). In our east/southeast locations of MN the probability for 1"+ is a bit higher than 24hr ago (now 60%).

Low probability severe potential Thursday (less than 5%):

Most ensembles favor a period of shortwave ridging in the wake of the departing mid/upper low before the next organized wave arrives Thursday night. There is a signal along the western axis of this ridging for SW return flow ahead of the next wave. This may result in increasing BL Tds and steepening mid level lapse rates. Deterministic GFS (Day 4) shows MLCAPE in the 1000-1700 J/KG range across our south and west during peak heating Thursday afternoon. At this range there isn't a strong signal in SB/MU CAPE from ensembles during the same period (7% prob for 1000 J/KG SB CAPE near SD/ND state line). Veered hodographs and effective shear in excess of 40kt as shown would support of organized convection/supercells "if" that pattern sets up and there is forcing available to initiate convection/overcome capping. There is no signal in ensemble based machine learning, analogues, and NBM CWASP. The lack of ensemble support lowers confidence enough to hold off on messaging a severe threat, but it is still worth monitoring.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/
Issued at 654 AM CDT Mon May 20 2024

Flight conditions remain variable this morning across eastern ND and northwest MN with several developing pockets of MVFR stratus and patchy shallow/ground fog. Conditions have varied from VFR to LIFR over short periods and while the fog should lift by 13Z stratus may fill in and persist before finally pushing east mid to late morning when prevailing VFR conditions become more likely. Winds are light and variable but should increase from the northwest 10-13kt during the daytime period as low pressure shifts east and high pressure builds to the north.



FGF WATCHES/WARNINGS/ADVISORIES
ND...None.
MN...None.

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