Scientific Forecaster Discussion

000 fxus63 kmkx 201814 afdmkx Area forecast discussion National Weather Service Milwaukee/Sullivan WI 114 PM CDT Thu Jun 20 2019 Update... Earlier stratus has scattered out to a stratocu deck across the region. It still looks like a stray shower or two may be possible in a few spots this afternoon, but the vast majority of the area will remain dry. && Aviation(18z tafs)... MVFR to VFR stratocu is expected this afternoon across the area. A few additional showers aren't out of the question, but if they do form, they will be very widely scattered. Winds will remain from the north to northeast. Winds will become light and variable tonight as high pressure noses into the area. The cu should dissipate with sunset, but mid and high level clouds will push in from the west. A little light fog is possible as well, though the increasing mid and high level clouds will likely temper that potential. Winds will become southeasterly on Friday, becoming breezy by afternoon. && Previous discussion... (issued 909 am CDT Thu Jun 20 2019) Update... A few light showers and areas of drizzle continue to pull east across the far southeastern portions of the forecast area. This precip should end completely by mid day. The clearing line right now stretches from Monroe to Madison to Sheboygan. Some new stratus has formed on the west side of the clearing, effectively keeping the cloud cover stationary despite an eastward trajectory to the showers. We may see this erode a bit later this morning, but suspect that any locations that see sun will quickly develop a cu/statocu deck. Additional mid and high level clouds are also approaching from the west, in association with another mid level shortwave. Bottom line--what clearing is out there right now may be somewhat short lived. With the approach of that mid level wave, a few additional showers are possible this afternoon, mainly over south central Wisconsin. Coverage and intensity look to be pretty limited at this point. Marine... Gusty north winds will slowly diminish today as high pressure builds into the area. Winds will eventually become southeasterly on Friday, with strengthening southeasterly flow into the weekend. A Small Craft Advisory may eventually be needed for the nearshore over the weekend, for both winds and waves. Several rounds of thunderstorms are also possible through the weekend, primarily affecting the southern portions of the lake. Aviation(12z tafs)... The low stratus is expanding inland, and so far there is no fog apparent on marine webcams or other webcams. The ceilings will gradually improve later this morning, with mostly VFR conditions by middle to late afternoon. A few showers may develop well inland this afternoon. We should have a period of mostly clear skies across southern WI overnight. Previous discussion... (issued 345 am CDT Thursday Jun 20 2019) Short term... Today - forecast confidence...medium. Low pressure over eastern Illinois and its associated shortwave trough will continue to move slowly eastward, reaching the lower Great Lakes this afternoon. The light rain will linger over southeast WI into mid morning before exiting to the east. Drier air will arrive on northerly winds over eastern WI and Lake Michigan and help to clear out clouds during the middle to late afternoon. The drier air progression may stall for a while, so expect broken cumulus or stratocumulus over south central WI for the day and we may be able to build a little cape due to steep low level lapse rates. The meso-models are showing isolated showers popping up within this weak convergence area. Added slight chance pops to account for this, but left out thunder for now. Tonight and Friday...forecast confidence...medium. We should have a period of clear skies tonight. Low temperatures will be in the lower 50s east and upper 50s well inland. Friday will begin mostly sunny. Then expect increasing mid and high clouds out ahead of the next system (and proposed decaying thunderstorm complex) approaching from Minnesota and Iowa. Steady east winds will keep temps cooler near the lake once again. Inland highs should be in the mid 70s. Friday night through Saturday night - forecast confidence...low to medium. Frontal boundary will be farther south Friday night with more short wave ridging holding sway across WI Friday evening. Hence was able to trim back pops in eastern areas. However need to keep pops in western County Warning Area and across the entire area for later in the night as combination of weakening low level jet and moisture advection spreads across southern WI, in addition to approaching mid-level short wave from the southwest. Precipitable water increases to 1.5 to 2 inches through Saturday morning. Hence will have higher pops for showers and storms later Friday night into Saturday morning. While showers and T-storms are likely this period, widespread heavy rainfall is not expected as the meso-Beta velocity will be above 35kts much of the night and Sat morning despite higher precipitation efficiency due to a deeper warm cloud depth. More concerned about a second night of heavy rainfall potential Saturday night into Sunday morning. Deep column moisture will be in place, with a deeper warm cloud depth. The concern is that the meso- Beta velocity looks less than 10 kts for a time per NAM BUFKIT soundings, while GFS soundings show faster speeds. Fortunately, southern WI is not on the nose of a strong low level jet. However other synoptic forcing affecting the area will be enough to warrant heavy rainfall and flooding concerns for Sat night into sun mrng. Of course, this will also be dependent on rainfall that occurs Friday night into Sat mrng. Long term... Sunday through Wednesday - forecast confidence...medium trending low. Medium range guidance in fairly good agreement on a passing vigorous low pressure short wave and sharpening low pressure trof looks likely to bring a period of showers and thunderstorms to southern WI from Saturday night into Sunday morning. The threat for precipitation will diminish later Sunday but not completely shut off as a secondary upstream low pressure area will be poised to move into the upper Midwest close on the heels of the weekend system. Although guidance is showing this upstream wave, it diverges as to the timing and strength of this system as it passes across the western Great Lakes early next week. GFS more progressive and weaker, with the precipitation focus mainly on Monday. The Gem and European model (ecmwf) show this low pressure area much slower and deeper as it moves across the region through Tuesday night. Due to discrepancies, wl need to carry low pops through early next week. Temperatures continue to look warmer for Sunday through early next week. && Mkx watches/warnings/advisories... WI...none. Lm...none. &&

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