EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1140 AM EDT SUN JUL 13 2014
BIG STORY CONTINUES TO BE THE ANOMALOUS COOLING IN THE CENTRAL PLAINS AND TOWARD THE EAST... WITH 850 TEMP STND ANOMALIES IN THE -3 TO -3.5 RANGE YIELDING SFC TEMPS ABOUT 20-30F COOLER THAN AVERAGE. DAILY RECORD COOL HIGH TEMPS SHOULD BE CHALLENGED IN MANY LOCATIONS.
Strong dynamics and cold air aloft should produce scattered showers and an isolated thunderstorm
mainly Monday but generally sinking air, limited instability and moisture will keep a cap on anything
severe. High temperatures at their coldest will be in the 60s; while lows dip into the upper 40s to lower 50s.
Look for more typical summer time temperatures that are closer to normal to return by the weekend.
Quick Update 7/12-----
General consensus of model and numerical guidance remains similar to that of last few days. Best chance for storms, possibly strong to severe, will be late overnight Saturday into Sunday morning. Variably cloudy skies with instability showers and isolated storms, along with gusty winds, can be expected Monday into Wednesday as the cold air aloft and at the surface overspreads Southeast Lower Michigan. Coldest temperatures still appear Tuesday into Wednesday with overnight lows falling into the upper 40s to lower 50s...and highs in the 60s.
Latest guidance from both GFS and Euro continue to amplify intense vortex diving out of Canada and into the upper Great Lakes states Sunday night through Wednesday. If anything, the latest GFS is a bit stronger with the system with very impressive wind max jet cores and cold temperatures aloft by summer standards.
As the colder air and front approaches Sunday night into Monday showers and thunderstorms will be likely and, depending on the environment instability, some could be strong to severe but at this early juncture strongest storm potential appear to be w and south of the region but this bears watching since close.
After; a high altitude jet stream core flirting with 140 knots is now projected by the GFS model Tuesday afternoon over the region! This max wind aloft is even notable for winter standards let alone summer. Of course it's early, but winds in excess of 100 knots seem likely anyway over the Great lakes region given the dynamics and strength of the system the past several runs. In addition; upper level heights /500 MB/ are now forecasted to hover near the all time record low for July ~ 558 dm over Southeast Lower Michigan recorded in upper air data.
Fortunately with limited instability in place, showers and isolated storms with moderate squalls would be expected. Winds closer to the ground in the 5 to 10 thousand denoting possible gusts range in the 30 to 50 mph, but again this is still up in the air - pun intended. How much wind is realized and the surface will be largely dependent on; how much sunshine and instability is realized, how much convection will be able to develop along with how much of the stronger winds are able to mix down to the surface.
Note the single digit 850 MB temps @ 5K FT Tue Eve
The large cool high pressure and air mass for mid summer extends far south to near the gulf states; a feat hard to come by in mid summer. Overnight lows over a huge portion for the country are in the 40s and 50s...more typical of early October weather.
Yes indeed; this Polar Vortex won't give up the ship in the normally warmest time of the summer!
Coldest of air over Southeast Lower Michigan still appears to be from Tuesday into Wednesday with possibly Thursday morning as well. Little change in original thinking as highest temperatures during Tuesday and Wednesday afternoon look to be in the upper 50s to mid 60s across Southeast Lower Michigan, while lowest temperatures in the mid 40s to the lower 50s.
We'll have more time to zero in on this system over the weekend.
Bill Deedler -SEMI_WeatherHistorian
Ironically, just over six months to the date of the first notable visit of the winter polar vortex (this is nothing new of course; happens every year but with varying intensities); she is expected to make a return visit mid summer. What IS impressive on this visit (not unlike her past winter visits) is the depth and position south. At early inspection, the upper low "polar vortex" is expected to average around 3 deviations BELOW average for a summer type upper low over the upper Great Lakes. Along with the fanfare of her arrival will be strong upper winds /jet stream/ and possibly some tough weather in the form of wind storms with such impressive energy at and above ground level. When such cold air (for summer standards) rudely plows into even temperate summer like temperatures (in this case 70s to lower 80s), something's got to give.
The two questions are when and where the best clash of the notably different atmospheres will be; which will be involving very impressive dynamics and instability levels at the time. It's much too early to give an exact timing but an estimate seems to be in the late Sunday into Monday time frame for it's arrival - and early - mid week when she spins up over southern Canada and northern Great Lakes. Even at this time, if available moisture is in place, it wouldn't take much to pop a gusty storm or shower with such cold conditions aloft along with the impressive strong winds.
Projections for upper heights and widespread low level cool temperatures are some of the best (lowest) I've seen for mid July. Upper low height projections are sub 550 MB /546 MB/ on the GFS along with 850 MB temperature projections in the single digits. At the coolest (preliminary timing; Tuesday night-Wednesday morning) widespread overnight lows in the 40s to lower 50s are likely across Southeast Lower Michigan with highs around 60 to the mid 60s. If these temperatures are realized, both record low maxs and record low mins may be in jeopardy. Record low max's for mid July are in around 60 to mid 60s across the region, while record lows are in the 40s to near 50. One glaring exception is a record low max of just 74 degrees in Flint on the 16th, much higher than other record low maxs from the 14th thru the 16th.
Its too early to predict the exact coolness of the air mass due over the area early-mid week during next week but model projections have been pretty well unanimous on this summer cool outbreak - it is coming with the particulars yet to unfold. If you are heading up north this weekend and plan to stay awhile pack for fall-like weather for a few days - LOL- really not a bad idea in Southeast Michigan too.
Bill Deedler -SEMI_WeatherHistorian