Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
599 FXUS63 KGRB 121134 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 634 AM CDT Fri Jul 12 2024 Updated aviation portion for 12Z TAF issuance .KEY MESSAGES... - Small chances (30% or less) for showers and storms this afternoon and evening. Severe weather is not expected, but storms could produce locally heavy downpours. - Warmer and more humid conditions arrive this weekend through early next week. Heat index values in the 90s are expected over central, east central and parts of far northeast WI Sunday into Monday. Heat indices could approach advisory criteria for a time in the afternoon across the southern CWA on Sunday. Thunderstorm development could also provide relief from the heat at times. - Increasing moisture and instability, combined with a couple short-waves and a cold frontal passage on Monday, will bring potential for a few rounds of strong to severe storms this weekend into early next week. Timeframe with the highest chance for the severe storms is Saturday night into early Sunday morning. && .DISCUSSION... Issued at 418 AM CDT Fri Jul 12 2024 Short Term...Today...Tonight...and Saturday Main forecast concerns will be if/where to include small shower/storm chances today and this evening, storm chances and severe potential on Saturday, fog potential and the increasing temps and humidity. Shower/Storm Chances & Severe Wx Potential: MUCAPE of up to 800 J/kg still present across parts of central and east central WI early this morning, so could see a stray shower pop up, but no clear trigger or area to really focus on. Will add some small PoPs over the east where a couple showers have popped up, but will keep things dry elsewhere. As surface instability increases through the morning, SBCAPEs will build to 1500 J/kg. Not a clear cut trigger through the day, but a weak shortwave and a lingering weak boundary or two, including the lake breeze, may be enough to touch off isolated showers and storms. A warm layer just above 700mb, weak ridging trying to build in and some mid- level dry air will be negating factors. Severe weather is not expected, but brief heavy downpours are possible as flow remain weak (slow moving storms) and PWATs remain around an inch. Soundings also showing an inverted-V again, but still not much wind to mix into. Any wind would have to come for a storm that can pulse up and generate some brief wind of its own. Most/all of the shower/storm activity will end in evening as instability wanes. For Saturday...many CAMs are now advertising a complex of storms approaching from the west early in the day, prior to 12z. But the evolution of this convection remains uncertain, as track/timing/ instability/strength differences remain. As evidence, huge differences from the 00z and 06z with two completely difference scenarios portrayed. Leaning towards the scenario that brings just isolated activity during the day on Saturday with an initial weak shortwave, either from a weakening complex and/or some pop up activity, as we will be fighting capping and dry air. Then we will have to wait until after dark for a larger complex to arrive from the northwest. Fog & Smoke Trends: Fog has been held in check for most of the area during the overnight hours as winds just off the surface are a little stronger than previous nights, some clouds have hung around and moisture is a little less over the north. But expect some patchy fog to continue to develop, mainly where skies are clearer across central and north central WI, through sunrise, then any fog would dissipate shortly thereafter. Fog is possible on northern Lake Michigan as well, with some dense fog just to our north early this morning (nothing seen yet on the Washington Island cams). Thinking the light south winds will push any fog bank north, but will monitor trends. Fog potential for tonight looks very low, but can`t rule out a little patchy ground fog early Saturday morning, especially if/where any heavier rain falls today. Quite a bit of elevated smoke was observed yesterday, thin in our area but thicker to our west. Smoke guidance has this area remaining across the region, with the thickest smoke remaining to our west/north. HRRR shows some thicker smoke pushing across Lake Superior and the U.P., which may sneak into the north. Other than hindering surface heating slightly and providing for a hazy skies at times, not expecting any other impacts from the smoke. Building Heat & Humidity: Above normal temps are expected as southerly flow will continue to allow temps and humidity levels to increase today into Saturday. Highs will mainly be in the 80s, with a couple spots possibly touching 90 in the south on Saturday if the clouds hold off long enough as 925mb temps climb to the 22-24C. Heat indices in the 80s are expected today, with the southern CWA likely around 90 on Saturday. Temps near Lake Michigan will remain slightly cooler. Long Term...Saturday Night Through Thursday The attention of the extended will be on the upcoming weekend to early week period which may see several rounds of active weather. Behind this, much quieter conditions move in for the middle of next week. Storms this weekend... Saturday evening, diurnal destabilization may have already led to the development of a storms to our west where the best instability gradient will be. Although bulk shear remains modest, around 20 knots, if these storms start to become outflow dominant they may yet make their way eastwards into portions of central WI. A better bet will be a fast moving shortwave that will cross the area in the early to mid overnight hours, crossing north-central to northeastern WI and the Upper Peninsula. The timing on this shortwave still varies between runs, from late evening to around midnight, but most mid-range models either initiate or sustain storms with its passage. The southern extent of any precipitation with this shortwave remains unclear at this time as well. The most likely severe threat will be wind with any storms that form, but a brief hail threat is also possible. Sunday will be largely influenced by any overnight convection from Saturday, which may leave a lingering outflow boundary in the region. Any outflow boundary will be something to watch for convection across the area on Sunday. Outside of this, capping does appear more modest than previous model runs suggested, which could mean an increased chance of convection during the day, especially for north-central WI where the warm layer is the least robust. Finally, another weaker shortwave cross the region late in the day on Sunday, which could serve to sustain some overnight precipitation. Monday will be another day to watch for severe weather. The warm moist airmass will still be in the region ahead of an approaching cold front. As things stand, this may prove to be the most organized of the three days, with afternoon and/or evening storms developing in the region. Temperatures...Outside of the storm chances, the warm and moist airmass will also produce a period of very hot weather for Sunday and Monday. Sunday will be the hotter day, with highs in the lower 90s and dewpoints potentially pushing into the middle 70s or higher. This would produce heat indices into the middle to upper 90s. After the cold front on Monday, cooler air will bring temperatures back towards the 70s for next week, which should also provide a period of quieter weather. && .AVIATION...for 12Z TAF Issuance Issued at 633 AM CDT Fri Jul 12 2024 Very patchy ground fog will mix out early this morning across central and northern WI, with a very low chance of some ground fog early Saturday morning. Otherwise, VFR conditions expected through the TAF period. Some daytime cumulus clouds are expected later this morning and afternoon, with some lingering clouds tonight. An isolated shower or storm is possible this afternoon/evening. Smoke may be present aloft at times. Winds will remain light. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Bersch/Uhlmann AVIATION.......Bersch