WEATHERAmerica Newsletter, Sunday, June 10, 2007 at 5:10 A.M. ET

[Larry_Cosgrove]

Jet Stream Heads North, Leaving Haze, Heat And Humidity In Its Wake (Mainly West Of The Appalachian Mountains); Tropics Look More Active, Gale Center Forms Off East Coast

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WEATHER SUMMARY THROUGH NEXT 72 HOURS

The one truly dynamic system on the weather map is the strong shortwave and cold front over the Pacific Northwest, which, as it moves its way through the northern tier states, could set off severe thunderstorms across southern Canada (into ON) and through the U.S. along and above an Interstate 80 line. The rest of the precipitation mechanisms are based on stagnant upper lows or weaknesses; see the convection over the Great Plains and the coastal Southeast as examples. Where it is not raining, however, is the sign of ridging aloft. As a subsident regime becomes entrenched across the Midwest and interior of the Old South, temperatures will rise, and greatly so.

The hottest air, of course, is currently over the Desert Regions. Part of the rebuilding cycle of the Sonoran heat ridge, the cT domain will likely expand into the High Plains and Missouri Valley within 72 hours, forming a convergence zone with the mP regime to its northwest and the warm, moist values situated from the south central U.S. into the Great Lakes. This may mean a better chance for strong to severe thunderstorms on Tuesday as energy aloft comes into contact with the air mass collision pattern.

A formative gale center off the Mid-Atlantic coastline is being handled poorly by the numerical models. The low covers a large geographic area, and is setting up a forcing configuration (mP vs. IcP) over the eastern slopes of the Appalachian Mountains and Atlantic Coastal Plain (generally along a line from Norfolk VA to Burlington VT). If enough sunshine is available, destabilization will occur and thunderstorms may form this afternoon and later Monday. As for the offshore low, many of the numerical models hold the circulation over or close to the Gulf Stream, within a 500MB weakness. Read the TROPICAL WEATHER OUTLOOK below for possibilities with this feature.


SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)

Some Thunderstorms May APPROACH Severe Limits
C CO....NM....TX....OK....KS....NE....E SD....W MN....W IA....MO....AR....LA

Some Thunderstorms May APPROACH Severe Limits
FL....GA....SC....W NC

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Landspouts)
NW NV....E OR....ID....W MT....S AB....S SK....S MB....W ON....Upper MI....N WI....N MN

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail)
VA Tidewater....MD Eastern Shore....DE....E PA....NJ....E NY....W CT....W MA....VT


HEAVY RAINFALL OUTLOOK
(potential for an inch or more total rainfall within the next 24 hours)

Isolated Locations In
C CO....NM....TX....OK....KS....NE....E SD....W MN....W IA....MO....AR....LA
(QPF 1-2")

Isolated Locations In
VA Tidewater....MD Eastern Shore....DE....E PA....NJ....E NY....W CT....W MA....VT
(QPF 1-2")

Isolated Locations In
FL....GA....SC....W NC
(QPF 1-2")

Isolated Locations In
NW NV....E OR....ID....W MT....S AB....S SK....S MB....W ON....Upper MI....N WI....N MN
(QPF 1-2")


EXCESSIVE HEAT ADVISORY
(potential for daily maxima above 95 F)

C, S CA....C, S NV....AZ....UT....SE ID....WY....S MT....SD....NE....KS....CO....NM....TX....OK....S AR....LA....MS....AL....FL....GA

MEDIUM RANGE OUTLOOK
(weather trends through the next 6 to 10 days)



As we slowly lurch toward "calendar summer" (and do not get me started on that "meteorological summer" nonsense), changes are evident in the 500MB longwave pattern that will favor the staples of a middle or late June forecast in the lower 48 states: heat, humidity, and isolated thunderstorms. A caveat in delivering the national forecast is the continuing chaos among the numerical models, which tend to show major disagreement after Day 7. I suspect that much of the problems of inconsistency (run to run, equation to equation) stems from recent changes in the GFS and ECMWF versions. With this issue in mind, it is probably best to use a good dosage of persistence and climatology in formulating the outlook through the following 240 hours.

A generalization that can be reached about computer depictions of the next ten days is that the polar westerlies (which have recently decreased in velocity and amplitude of standing waves) will tend to stay just above the U.S.-Canada border during the foreseeable future (that is, through the next two weeks). Ensemble means and derived anomalies strongly suggest some very hot, humid conditions through the Midwest and Old South. A Sonoran heat ridge is shown on many of the variants, with another subtropical high extending from the Great Lakes into FL. 500MB weaknesses (favoring dewpoint advection and diurnal thunderstorms) are favored just west of the Mississippi River and over the Gulf Stream. The latter trans-ridge gap may contain the gale center which is currently situated off of the Eastern Seaboard, which implies at least a small danger of extratropical conversion.

While afternoon and evening convection may be locally heavy in parts of the nation to the right of the Rocky Mountains, the greatest risk of severe weather will be through southern Canada and the northern tier states, closest to the polar jet stream and the impulses ridging along it. Initially, ridging in the Great Lakes will eliminate the potential for thunderstorms, but after 192 hours the energy will reach E ON, QC, and New England. So between June 18-20, threats for northwest-flow thunderstorms could increase in parts of the Northeast. One of the disturbances could be accompanied by a fresh intrusion of cP values, offsetting the expected intense heat building into the Ohio Valley.

TROPICAL WEATHER OUTLOOK
(trends across the Atlantic and Pacific Basins)

Judging by the look of weather satellite imagery over and near the tropics, the 2007 Hurricane Season may live up to its billing as "active" and "exciting".

So far, the appearance of disturbances across the lower latitudes of the Northern Hemisphere is akin to what one would expect in a weak-to-moderate La Nina episode of ENSO. Now while there is some debate about the "cold" or "warm" forecast for the equatorial Pacific realm (will the status be neutral or a full-blown La Nina), we can clearly see that the lower latitudes west of Baja CA are dominated by westerly winds aloft, with the flow signature over the Atlantic Basin showing signs of strengthening of deep mean easterlies. There are two prominent tropical waves in view (Panama and west of Sierra Leone) which sport circulations about a convective core. Also noteworthy is the fact that there are two gale centers in the mid-latitudes; one near Portugal appears to be fading, but the gyre east of MD is sinking south and is strong enough to cause forcing over sections of Appalachia and the Eastern Seaboard (see comments above). There is a small chance that, with rising 500MB heights over the interior of North America, that the low will drift further south, and like Andrea before it, acquire subtropical characteristics. I write of this concern since many of the NWP schemes stall the low out over the Gulf Stream in an upper level weakness.

The disturbed area over Panama may intensify, but odds are this system will do so over the warm waters of the Pacific Ocean below Central America. Since light southwest (shearing) flow is forecast to develop west of the Gulf of Tehuantepec, the impulse may face the same fate as Alvin and Barbara before it (sheared apart over cooled, upwelled water). As for the impressive convective circulation below the Cape Verde Islands, the relative coolness of the equatorial Atlantic waters argue against development. Still, it should not be long before some of the west African waves have a shot at reaching the Lesser Antilles under warming seas and the already favorable wind environment.

Prepared by Meteorologist LARRY COSGROVE on
Sunday, June 10, 2007 at 5:10 A.M. ET

Disclaimer:

The previous statements are my opinions only, and should not be construed as definitive fact. Links provided on this newsletter are not affiliated with WEATHERAmerica and the publisher is not responsible for content posted or associated with those sites.

Copyright 2007 by Larry Cosgrove

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