Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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724 FXUS63 KGRB 040312 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1012 PM CDT Wed Jul 3 2024 Updated aviation portion for 06Z TAF issuance .KEY MESSAGES... - Next round of widespread showers/storms arrives from the west Thursday afternoon/evening, becoming more widespread overnight. Confidence is increasing these showers/storms will impact fireworks displays the night of the 4th, especially across central and north-central Wisconsin. - Threat for flooding is increasing Thursday evening into Friday from this next round of heavy rain. Rivers and streams will likely rise, with some urban/rural flooding also possible. - The unsettled weather pattern continues Sunday through (at least) Tuesday with on and off chances for rain/storms. && .DISCUSSION... Issued at 347 PM CDT Wed Jul 3 2024 Heavy Rain Threat / Precip Chances: Showers have developed across northwest and north central WI early this afternoon, so will continue with isolated showers this afternoon and evening, especially over northern WI, as we reach peak heating and the robust cumulus clouds track eastward. Still can`t rule out a rumble of thunder as instability of nearly 1000 J/kg and steep low level lapse rates will continue this afternoon. Dry conditions are expected later tonight into Thursday morning. Attention turns to the arrival of the next round of showers and storms on the 4th. Deterministic and CAMs still showing slightly different scenarios, especially the HRRR which seems to be a fast/more aggressive outlier. Would like to ignore this solution, but it has captured some events in recent weeks, so won`t completely throw it out. Leaning toward a model blend, have only made minor changes to the going forecast for the 4th, bringing in chance PoPs from west to east through the day, with one round possible in the later morning and afternoon associated with a push of some elevated instability and weak warm/moist advection, then the main round in the evening into the overnight as the main shortwave, low pressure and upper trough drop across the region. Still some uncertainty on just how progressive this system will be, which will likely determine how much rain we will get. While PWATs won`t be as tropical as yesterday (when we had spots over 2 inches), still respectable values approaching 1.5 inches late tonight into Friday will be supportive of additional heavy rain. Additional rainfall of an inch or two is looking likely (40-70 percent of an inch or more, highest in central WI) from Thursday afternoon into Friday evening, which could aggravate flooding issues. Considered a flood watch for parts of the area that have received the most rainfall the past 2-3 days, mainly central and east central WI, as the continued bouts of heavier rain have led to flash flood guidance (FFG) to decrease and the overall flood threat to increase with each event. Parts of the Fox Valley have 1-hour FFG around/just under an inch, with 6-hour FFG between 1-2 inches. Parts of central WI are closer to the 1.5-2.5" range. The pockets of heaviest rain have been in different locations over the past few days and there has been at least a day or two of dry conditions to help rivers/low drainage areas to lower. With the heaviest rain still 24+ hours out, and some uncertainty on where the heaviest bullseye will be, will opt to hold off on any flood watch at this time but continue to highlight the flood threat in the HWO. The threat for severe weather will be low, as the better surface instability and effective shear will remain to our southwest. But some of the stronger elevated storms could produce some hail. SPC did maintain a marginal risk for severe weather in the far southwest forecast area. Saturday is looking dry as high pressure slides across the region. Then the pattern turns unsettled again Sunday into at least Tuesday as another large trough slowly rotates across the western Great Lakes. Some bouts of heavy rain will once again be possible with PWATs climbing back over 1 inch, possibly creating additional flooding concerns. Winds / Fog / Temps: Winds have struggled a little today as mixing was a little hampered in spots, but west winds could gust to around 25 mph the rest of the afternoon, then as mixing decreases, winds will decrease toward sunset. Lighter winds are expected tonight into Thursday. Patchy ground fog will be possible late tonight as winds die off, but SREF/HREF showing widespread fog potential under 5%. There will be additional fog chances during the overnight and early morning hours, especially after the heavier rain events and if/when skies clear. Above normal temps will continue into Thursday, then a brief cool down arrives on Friday. Temps rebound close to normal for the weekend and into next week. && .AVIATION...for 06Z TAF Issuance Issued at 1002 PM CDT Wed Jul 3 2024 Mainly VFR conditions expected through Thursday afternoon, with deteriorating flight conditions occurring over north central and central WI Thursday evening. Clear skies, light winds and wet ground from recent rainfall could lead to patchy ground fog overnight, but confidence is too low to add to the TAFs. An approaching low pressure system and warm front will bring a chance of showers and storms to mainly central WI late Thursday afternoon, though timing is still a bit uncertain. More widespread showers and storms should overspread the region Thursday night, possibly reaching the eastern TAFs by late evening. Light west to southwest winds will become southeast to east Thursday afternoon and evening. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Bersch AVIATION.......Kieckbusch