Sunday, March 6, 2016

Does Lowe's sell gopherwood? Flooding rain potential this week

A gorgeous weekend is in the books as high temperatures reached 70 degrees both Saturday and Sunday with a good deal of sunshine. Warm weather is here to stay, but the sunshine will be replaced by clouds, increasing wind, and, by mid-week, rain measured in inches.

Starting the week off, dry and warm conditions are expected Monday and most of Tuesday as high pressure off the Atlantic coast brings in warm southerly wind and a storm system out west begins to set up. This will mean occasionally gusty wind and above average temperatures with highs in the lower 70s and lows in the 55-60° range.

To our west, an unusually deep (strong) upper level low pressure system will eventually "cut off" from the main air flow across the country and sink south into Mexico, where it will "sit and spin" for a couple of days (see first graphic below). At the same time, the jet stream level wind, which typically blows west to east across the nation will buckle. It's common for ridges and valleys to form in the jet stream, but in this case a valley will become a cliff! As the wind moves into the western U.S., it will dive south around the anomalously low pressure in Mexico, then shoot back to the north over the Southern Plains (see second graphic below).

Anomalies, represented as standard deviations, in the upper level pressure pattern (18,000') on Wednesday morning. Unusually low pressure over Mexico (6 standard deviations below normal) will feed abundant tropical moisture north on the eastern side of the low into the south-central U.S. and eventually the Mid-South. Graphic courtesy WxBell.

The jetstream pattern on Wednesday morning is also highly unusual pattern with a low pressure area over northern Mexico and "meridional flow" (north-south, rather than west-east) over the center of the country. The jet stream from the south will transport record-breaking atmospheric moisture north into the eastern Plains, shifting east towards the Mid-South later in the week. Graphic courtesy WxBell.

The graphics above are effective Wednesday morning, but the general pattern will remain in place for a couple of days, meaning that whatever weather pattern they create at the surface will be stagnant. Unfortunately, that pattern for the south-central U.S. into the lower Mississippi and Ohio Valleys will be a wet one. The upper low over Mexico will serve to pull abundant amounts of atmospheric moisture north, transported by the south-to-north jet stream. Under the jet stream, and with possibly record-breaking amount of moisture in place, rainfall will be heavy and persistent, leading to flooding concerns across LA, AR, and southern MO Wednesday, and periods of heavy rain in the metro by Thursday. Bouts of severe storms are also possible the first half of the week, mainly southwest of the Mid-South.

Atmospheric moisture is depicted using precipitable water values (PWAT) for Wednesday morning. An "atmospheric river" of moisture that leads from the tropics into the central U.S. will feed tremendous rainfall amounts under it's core. Due to it's location, this atmospheric river has been dubbed the "Mayan Express" (similar to the "Pineapple Express," which feeds moisture from the central Pacific near Hawaii into California during periods of heavy rain there). Graphic courtesy WxBell.
By late in the week, the upper low over Mexico will finally start to move, but it will head northeast across eastern TX into AR and the Mid-South. This means that even though the flooding threat should diminish, an unsettled pattern will remain in place for the metro with periods of showers and thunderstorms all the way into next weekend. 

Rain chances will generally drop a bit each day after Thursday, but we'll still be dealing with decent chances of rain and thunder each day through next Saturday, and possibly Sunday. It also means that we'll remain in a warm pattern with highs remaining near 70° and lows in the upper 50s or warmer as surface moisture levels (dewpoints) remain high. Rainfall totals this week could be over a foot in parts of LA, 8-10" north into central AR, and a very large area of 3-5" totals, including the metro. 
Forecast rainfall totals through next Sunday morning from the NWS. Totals above 5" are evident west of the Mississippi River, which 3-5" is expected in the metro. Graphic courtesy WxBell.
Areas that typically experience high water with heavy rain will likely see that occur this week, especially on Thursday and possibly into Friday. In addition, Mississippi River water levels are already running higher than normal. With excessive rainfall upstream from the Mid-South, I expect those with interests tied to the river will need to continue to monitor water levels through March.

Stay tuned for later forecasts and be prepared for possible flooding late this week. We'll have daily updates either on this blog or our social media feeds listed below.

Erik Proseus
MWN Meteorologist

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