Scientific Forecaster Discussion

000 fxus63 kgrr 222000 afdgrr Area forecast discussion National Weather Service Grand Rapids Michigan 300 PM EST Fri Feb 22 2019 Latest update... synopsis/discussion Synopsis... issued at 300 PM EST Fri Feb 22 2019 - rain and a few thunderstorms late Saturday and Saturday night - very windy Sunday into Sunday night - colder with snow showers Sunday afternoon through Sunday night into Monday - below normal temps for most of next week && Discussion...(this evening through next friday) issued at 300 PM EST Fri Feb 22 2019 Primary short term fcst concerns continue to involve assessment of just how windy it will become Sunday into Sunday night. We also need to assess convective potential Saturday evening and potential for snow and any accumulations Sunday through Sunday night. Tranquil wx will continue late this afternoon through tonight into Saturday morning. A strong low level jet and increasing low level moisture transport will result in development of pcpn late Saturday morning/early Saturday afternoon. Thermal profiles by then will have moderated sufficiently enough for pcpn to fall as plain rain across roughly the southern two thirds of our fcst area. However over the northern third of our fcst area pcpn may begin as a brief period of light freezing rain with a few hundredths of an inch of ice accumulation possible over our far NE fcst area up toward Clare. The rapidly strengthening low pressure system will bring rain and a few thunderstorms Saturday night as h8 Li/S go slightly negative (-1 to -2) with two to three hundred joules of MUCAPE. Peak elevated instability occurs between 00z sun and 06z sun. Severe wx is unlikely given lack of much stronger instability. However some convection could bring a few higher end wind gusts given how strong and dynamic of a system this will become and the magnitude of winds aloft. It will become very windy Sunday as the sfc low continues to deepen rapidly as it moves NE. A consensus of latest computer model guidance continues to suggest wind gusts across our fcst area will reach 50 to 60 mph. It is noted that there are a few guidance solutions such as the operational 12z European model (ecmwf) and 12z European model (ecmwf) eps are not (quite) as bullish with peak gusts as previous runs. We will continue to monitor guidance trends closely. All things considered though given how quickly the low is forecast to intensify (995 mb low north of kstl at 00z sun to 970 mb low between ssm and James Bay by 00z mon) and that it is strong cold air advection we still are confident that gusts of 50-60 mph will be the most likely outcome for our fcst area Sunday. In addition rain will quickly change over to snow showers Sunday morning as colder air begins to advect in. A consensus of latest guidance shows snow showers will then continue to develop through the day and Sunday night as h8 temps plummet to around -14 to -16 c by 12z Mon. However the overall setup does not look favorable for significant les accumulations given the magnitude of wind speeds and orientation of the upper jet and with inversion heights fcst to crash to around 3 kft agl by 12z Mon. Therefore we are only forecasting snow accumulations of around an inch or less for most of our fcst area Sunday through Sunday night. A few locations over our northern counties may receive up to around two inches of accumulation. A broad sfc high pressure ridge will bring fairly tranquil but cold wx through the upcoming week. A northern stream system may bring some light snow Wednesday night into Thursday. && Aviation...(for the 18z tafs through 18z Saturday afternoon) issued at 1110 am EST Fri Feb 22 2019 VFR conditions will continue this afternoon through tonight and Saturday morning. Conditions will then slowly deteriorate to MVFR Saturday afternoon due to development of low clouds as well as some patchy fog. Light rain showers will develop around to shortly after 18z Saturday. && Hydrology... issued at 300 PM EST Fri Feb 22 2019 A significant warmup as well as rainfall expected this weekend will start the process of river levels rising again. Anytime we think about rapid warmups and rainfall this time of year leads to the logical question about flooding potential. The good news is that at this point it looks like the rain totals will be low enough and the warmup will be brief enough to avoid widespread and significant flooding across our area over the next week. However, significant rises are very likely on virtually all of our rivers, and by the first half of next week many of our rivers will again be near bankfull. We will keep a close eye on forecast trends, but right now it looks like our rivers should escape without major issues. The one fly in the ointment is the renewed threat of ice jams on some of our rivers. The last big rise on our rivers a few weeks ago destroyed a lot of the river ice, which will now work to our advantage (you can't have an ice jam without ice). Limited amounts of new ice have formed on parts of the rivers, but most of our rivers remain mostly ice-free at this time. However, there are some rivers that still have solid ice covering them, including parts of the Grand River (portland, Grand Rapids area, and Eastmanville/Robinson township). These areas will be the focus for potential ice jam formation over the next week, and should be watched closely. Once the upcoming warm and windy weather passes, temperatures will again plunge for next week, which will limit additional snowmelt and start to give the rivers a chance to recover from all the new water. && GRR watches/warnings/advisories... Winter Storm Watch from Sunday morning through late Sunday night for miz037>040-043>046-050>052-056>059-064>067-071>074. && $$

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