FXUS63 KGRR 032127 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 527 PM EDT Tue Aug 3 2021 LATEST UPDATE... Update .SYNOPSIS... Issued at 228 PM EDT Tue Aug 3 2021 - Isolated showers and thunderstorms early this evening - Risk for storms Friday into Monday - Warming up for the weekend into early next week && .UPDATE... Issued at 527 PM EDT Tue Aug 3 2021 I have increased to pop to likely north of I-96 and east of US-131 till around 9 pm this evening for locally strong thunderstorms with very heavy rainfall (locally over a inch of rain possible). Gusty winds and small hail are possible too. I significantly increased the rainfall in our QPF grid for this evening to over a half inch in our northern 2 rows of counties, near and east of US-131. The storms have become more concentrated and strong since 4 pm this afternoon. Radar storm totals are as high as 2.5" NE of Baldwin. The highest actual rainfall from a rain gauge is .60 inches at Goose Lake (near Tustin). Due to light winds aloft, that is winds below 30000 ft are less than 15 knots. The storms are slow moving, so locally heavy rainfall has already happened and will likely continue until around sunset. Model sounding show surface based capes are between 1000 and 1500 j/kg with an equilibrium level near 35000. The SPC SREF shows surface based cape rising to near 2000 j/kb by 7-8 pm. The are of higher capes is north of I-96 and while that area does move south over the next few hours, it only moves about 1 row of counties south. The storms seem to be on a shear lobe on the north side of upper low , which is centered near Cleveland now.That upper low drifts slowly east southeast so the storms will too. I expect the stronger storms will stay near and east of US-131 and north of I-96 this evening, then totally dissipate after sunset. Due to very low deep layer shear the storms will not be severe but due to higher cape and rather moist atmosphere these storms are in small hail, wind gusts to near 40 mph and locally very heavy rainfall should be expected from these storm. Isolated showers are expected near US-131 along the lake breeze into this evening but there is not near the cape near and south of GRR and the air aloft is drier too, so it will be hard to develop deeper storms south of I-96. && .DISCUSSION...(This evening through next Tuesday) Issued at 228 PM EDT Tue Aug 3 2021 - Isolated showers and thunderstorms early this evening ML Cape values have risen up to around 250 to 500 J/kg mainly north of a Grand Rapids to Mt Pleasant line. A few thunderstorms have developed along and east of the lake breeze in this region. They have been pulse in nature and will likely remain that way given the low deep layer shear values. As we approach sunset the convection will slowly diminish. We will continue to hold on the low POPs for that region through the early evening hours. - Risk for storms Friday into Monday One low level jet moves in Thursday night into Friday. Elevated instability starts showing up Friday. Meanwhile a series of weak upper level troughs move through through this period. As the moisture and instability increases through the period...so will the risk for storms. The first trough arrives Friday and could lead to a few storms. A second trough migrates through for later Sunday into Monday. Diurnally driven showers and storms are possible Saturday into Sunday...but the added lift from the second trough may bump up the potential for Monday. Deep layer shear is shown to remain relatively low so organized severe weather does not look likely at this time. - Warming up for the weekend into early next week With a general west to southwest flow prevailing into early next week...this will support climbing temperatures. Temps at 925/850 mb climb steadily Saturday into Tuesday. Above normal surface temperatures are expected for this period. Based on the 925/850 mb temps we should see highs in the mid 80s Saturday and upper 80s for Sunday into Tuesday. There could even be a few values reaching 90. There are some indications that it will be even warmer for the middle part of next week. && .AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon) Issued at 143 PM EDT Tue Aug 3 2021 VFR conditions will prevail through Wednesday morning. Some widely scattered showers are possible this afternoon and may have isolated areas of MVFR conditions but these were not included in the forecast due to low chance. Winds will be mostly light and variable. && .MARINE... Issued at 228 PM EDT Tue Aug 3 2021 A low level jet moves in for Friday and that will cause the waves to build. Right now we have values under criteria for headlines...but this will need to be monitored closely. Then for the second half of the weekend...another low level jet moves in and could support waves building close to headline criteria. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. LM...None. && $$ UPDATE...WDM SYNOPSIS...MJS DISCUSSION...MJS AVIATION...Ostuno MARINE...MJS