Monday, April 13, 2015

April showers - and a lot of them!

What a glorious spring weekend we just experienced in the metro! A good deal of sun, cool mornings, warm afternoons... oh yeah, and lots of pollen (bless you!). The pollen won't have much time to build up this week as the overall weather pattern looks very unsettled with multiple opportunities for rain and some thunder. Fortunately, severe weather doesn't appear to be a big concern.

A cold front is passing through the Mid-South this evening and will stall out just to our south. In the meantime, upper level systems and their attendant surface lows will track through a very moist atmosphere, triggering periods of rainfall. First, tonight, the cold front and an upper level system will move through bringing rain and embedded thunderstorms. Some of the rain will be heavy, with precipitation totals of 1-2" possible across the metro.

Frontal positions and rain chances around midnight tonight. A cold front will be just to our south with rain ending overnight.
Precipitable water values, which shows the amount of available moisture in the atmosphere, are near 2" in the Memphis area this evening, which is anomalously high for this time of year. This indicates the possibility of heavy rain with tonight's rainmaker. Graphic courtesy WxBell.
Later tonight, that front will stall to the south and remain there through tomorrow. With no major upper level systems moving through, rain chances will be slim as a northeast breeze brings in slightly drier air. A weak system will travel along the front on Wednesday. Depending on exactly where that front sits, we could see enhanced rain chances again Wednesday into Wednesday night, though there is some disagreement in the models about exactly how high those chances are. For now, we have 40-50% chances of rain. 

Another lull might occur Thursday with slightly reduced rain chances once again, but then it appears that we get back into a wet pattern to end the week and start the weekend. A large upper level low pressure system is forecast to move out of the western U.S. and across our area. Again, there is disagreement in the models on how fast it does so and what the ramifications are, but there is sufficient evidence to believe that Friday could be another wet day and that the entire system may not move out until late in the weekend. Thus we remain in an "unsettled" pattern for the better part of the coming week.

A large "bowling ball" upper level low pressure system is forecast to be over the southern Plains by Saturday evening, according to this morning's GFS model. Once this upper low moves through, our rain chances should diminish considerably. Models vary on how long that takes though. Graphic courtesy WxBell.

A word to the wise on this week's forecast: the timing, track, and strength of the features that will bring our rain chances are hard to pick out more than about 48 hours in advance, even though the general pattern is fairly well understood. Rain chances 3-5 or more days out likely WILL change. We invite you to check the MWN Forecast regularly as anything past about 2 days is fairly low confidence at this point. 

What we do know is that when all is said and done, we will have gotten a fair amount of rain by the end of the weekend ahead, perhaps as much as 3-5". Flash Flood Watches may be needed for later in the week if a system looks like a particularly heavy rainmaker and early week systems have saturated the ground and caused streams and creeks to rise. On the bright side, the pollen should get washed away multiple times this week!

Forecast rainfall amounts over the next 7 days are significant for areas south of the Ohio River to the Gulf Coast. NOAA predicts as much as 3-5" in the Mid-South by next Monday.
Erik Proseus
MWN Meteorologist

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