Friday, July 11, 2014

Temps are the big story as the upper-levels flip-flop

High pressure aloft and at the surface this weekend will mean hot temperatures with increasing humidity, leading to what could be the hottest day of the year so far on Sunday. With minimal rain chances and light wind, high temps will reach the mid 90s Saturday and Sunday, and perhaps Monday as well. Sunday has the best chance of being the hottest day - we have a forecast high of 96 with heat indices approaching the danger level (which is 105). Heat safety tips will be important this weekend and sunscreen will be a must if you spend any time outdoors.

That all changes next week as a system some are erroneously labelling the second coming of the Polar Vortex drops south out of Canada into the eastern U.S. We won't get into technicalities of the name of it (we firmly believe this is not a polar vortex though) [Read more: "Poor Man's Polar Vortex", "NWS Walks Away from Polar Vortex Claim," and "Cold Snap II: Revenge of the Polar Vortex?"]. However, the massive - and rare for climatologically the hottest part of the year - upper-level trough will be more welcome this time around than when the first "polar vortex" showed up in January with bitterly cold temperatures. and "Well below normal temperatures in July are simply dubbed "pleasant!"

Graphics from the GFS computer model (which are backed up by previous runs of the model and other medium-range models not shown) are below, but suffice it to say, record lows near 60 and record low maximums in the mid 70s are not completely out of the realm of possibility! Yes, in JULY! Note that all graphics are from a particular model solution and do not represent FORECAST conditions, but one possibility that is bolstered by multiple runs of the GFS model, as well as other medium-range model output.

The transition from hot to well below normal will be marked by a strong cold front that is expected to bring thunderstorms. Anytime this drastic of a change is in the cards, we have to keep our eyes open to the potential of severe weather. The time to watch appears to be late Monday into early Tuesday. We'll keep you posted. In the meantime, get some swimming  in this weekend and stay cool and hydrated, then get ready for a welcome sign of early fall by mid-week! Here's our latest forecast.

Average 500mb (18,000') pressure pattern from Sunday-Friday with anomalies (departures from normal) in color shading. The entire U.S. east of the Rockies has anomalously low pressure as a massive trough (dips in the black lines, or isobars) sits overhead. Troughs typically bring below normal temps, while ridges, like the one over the western U.S., bring above normal temps.
All following graphics show "departure from normal (DFN)" temperatures from the Friday morning GFS model. This one shows the cold air, represented by Sunday afternoon DFN high temps, from Canada south to the Ohio River.

DFN high temps for Wednesday as the broad upper-level trough settles over the eastern U.S. Note the 30-degrees below normal temps over the Plains, while the metro is 10-15 degrees below normal.

DFN high temps for Friday. The below normal temperatures stick around through the end of the week.

DFN low temps for Tuesday morning. Lows from 15-20 degrees below normal are found in the Central Plains while the cooler air seeps south into the Mid-South.

DFN low temps for Thursday morning. Lows in the metro of 10-15 degrees below normal (which is 74) could put us in record territory. 
Erik Proseus
MWN Meteorologist

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