The deep low which brought all-time record May snowfall from Wisconsin to Arkansas and record cold all the way to Houston is now a ‘cut off’ upper low and presenting the Southeast with flooding rains. Eventually this thing will clear out but another system is now driving into California and has cut the heat and should bring appreciable showers and thunderstorms through arid Southern California. That low will then advance east across the country.

Firstly, here’s the 5-7 day rainfall totals off the QPF. Note the 5+ inches of water falling over the Southeast.

Below is the 500mb chart off the GFS for today and you can clearly see the deep low spinning over the South but cast your eyes to the West Coast and notice the next system already pushing in.

[s2If current_user_can(access_s2member_level1)]

As this system dives into the Southwest, this should boost heights and increase an easterly flow from the Great Basin towards the coastal Pacific Northwest, so we’re looking for warmer conditions here with the possibility of record temperatures from Seattle down through Portland, perhaps extending as far south as Medford and Crescent City.

Here’s the 500mb out by Tuesday and notice the very slow forward movement of the upper low over the Southeast. That’s because of the amount of amplification or blocking within the pattern. Another reason for such high rainfall totals. We could easily see over 10 inch rain amounts over the southern Appalachains from this over the next 72+ hours.

Even by Wednesday the upper low remains over Georgia and the Carolinas with flooding rains spreading north into the Virginias while the Southwest low presents the Desert region with cooler temperatures along with showers and thunderstorms thanks to the instability created by the cold pocket aloft. Strong, gusty winds are also likely with this feature.

Towards late week, the SE low spreads north with it’s rains while in the Southwest, the upper low eventually fills as upper heights build and temperatures warm, providing the cap for unlimited sunshine and strong surface heating over the Desert.

So, to summarise the next 7 day period. The upper low which brought all the record cold and snow will remain a dominant aspect of the weather over the South and Eastern half of the country right through the week with flooding a serious threat, particularly from central Georgia up into the southern Appalachains. Rain, wind and cooler temperatures arrive into Southern California and will enhance shower and thunderstorm activity across the Desert region through much of the week.

What about the period beyond next weekend?

A negative NAO/AO and positive PNA signal is looming in the 7-10 day and even the 10-20 day with a couple of troughs taking aim at the Eastern two thirds of the country while ridging is set to take centre stage Out West as you would expect at this time of year.

If you live anywhere from the Upper Midwest southeastwards into the Ohio Valley, across the spine of the Appalachains and into the interior Coastal Plain (west of the major cities), one must be concerned about the prospects of a mid to late month frost and freeze threat.

Both the ECMWF Deterministic and Control are hinting at another fairly significant cold trough dropping into the Dakotas and Minnesota by next Sunday onwards which could threaten unseasonably cold days with a frost and freeze by night as a Canadian high drops down. Keep in mind that Canada is STILL unseasonably cold from Manitoba east into central Ontario and this along with a a still largely drozen Hudson Bay, these cold shot which come down, could still pack a punch despite this ridicuously late time of year.

Here’s the ECMWF deterministic by next Sunday.

The period from next weekend through the middle part of the following week (10-15th), shows the positive PNA signal as a ridge builds north, with it’s axis centred over the CA-NV line and pumps heat all the way up into northern British Columbia which in turn drops the trough back down into the Northern Plains. With time, the trough and chill spreads east, possibly getting as far down as the Tennessee Valley around the 13-14th.

Here’s a look at Tue 14

Notice the ridge begins flattening over western Canada and spreads the heat east through the Great Basin but it also is seen to deepen the eastern trough, driving cooler air right into the Southeast. Could this threaten a VERY LATE frost all the way into the mountains of Tennessee, Georgia and the Carolinas.

While temperatures may run 10-25 below normal from North Dakota to eastern Tennessee and possiblity the Carolinas between the 10-15th of May, the heat is very much on from coastal California east through the Deserts and all the way up to western Canada with record highs likely to fall for the same period.

Below is a comparison between the GFS and ECMWF in the 500mb heights in the 8-10 day period.

Notice the ECM has a much stronger/colder polar vortex over Hudson Bay with a deeper trough down into the Eastern US. It also has the western ridge positioned further west and centred back over the Pacific Northwest, rather than over southern Saskatchewan like the GFS has.

Looking back to the day 10 period, both the deterministic and Control flatten the eastern trough with warmer air spreading east through the central Plains into the Ohio Valley but by the time it gets into the East, it’s moderated and not particularly warm. In other words, the 80s and perhaps 90s which get out onto the Plains, will cool to the 70s, low 80s over the Ohio Valley, possibly limited to the low 70s east of the Appalachains, the real warmth never gets much beyond the Mississippi River..

Here’s the GFS 8-14 day temperature probability.

Interestingly, beyond the 15th, the Control shows another trough dropping out of the Great Lakes into the Eastern half of the US. This suggests another spell of below normal from around the 15-20th.

There is no doubt, the way thiungs stand right now, the heat will be focused in the West this month with temperatures likely to end up below normal from the Dakotas down towards Florida this month.

[youtube http://www.youtube.com/watch?v=J2e_OFpETZQ]

[/s2If][s2If is_user_logged_in() AND current_user_cannot(access_s2member_level1)]

That’s it, [s2Get constant=”S2MEMBER_CURRENT_USER_DISPLAY_NAME” /]!

[warning]To continue reading, you need to have a valid subscription to access premium content exclusive to members. Please join a subscription plan if you would like to continue.[/warning][/s2If][s2If !is_user_logged_in()]

Sign in to read the full forecast…

Not yet a member? Join today for unlimited access

Sign up to markvoganweather.com today to get unlimited access to Mark Vogan’s premium articles, video forecasts and expert analysis.
[/s2If]