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WEATHER HAZARDS (During The Next 24 Hours)
SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
Some Thunderstorms May APPROACH Severe Limits
(Pressure Gradient Derived, Orographically Enhanced)
(Pressure Gradient Derived)
HEAVY RAINFALL OUTLOOK
(potential for an inch or more total rainfall within the next 24 hours)
Isolated Locations In
(QPF 1 – 2″)
WINTER WEATHER OUTLOOK
(potential for maximum temperatures below 25 deg F within the next 24 hours; snowfall in excess of 4 inches over a 24 hour period; or other frozen precipitation exceeding trace amounts)
Scattered Locations In
W, C ON….N MN….E ND….S MB
(Snow, Sleet; 4 – 8″)
Scattered Locations In
W AB….S BC….WA….ID….OR….CA….NV….AZ….UT….W CO….W WY
(Snow; Mainly Above 3500 Feet; 4 – 12″)
GLOBAL WEATHER SUMMARY
(a review of important weather features around the world)
A thunderstorm cluster to the immediate east of the Philippines has potential for strengthening into a tropical cyclone. Since the Madden-Julian Oscillation is no longer imparting energy to the polar jet stream (convection associated with the MJO has retrogressed somewhat into the Indian Ocean theater), cold advection is likely to relax over eastern Asia during the near term. Downstream effects in North America can be expected as well, with the Arctic air mass now entering the U.S. having a shorter duration of existence.
Obviously, the “Big Daddy” storm complex centered off of the coast of Oregon is the major player on this satellite image. The cold dome and wind field about the disturbance stretches in a radius of about 1500 miles. Two complications arise in forecasting the full impact of the trough. One is the couplet of impulses approaching from below Alaska and the Aleutian Islands, which promise to have major impacts on apparent weather in the lower 48 states between November 23 and December 2. The other is the subtropical jet stream, which has lost its connection to the Madden-Julian Oscillation (signal fade in the western Pacific Ocean) but still will bolster the two lows moving toward the U.S.
The prominence of the huge storm complex along the West Coast is accentuated by ridging and warm advection to the right of this system across the eastern U.S. Note that convection across South America is channeling northeastward, with widespread thunderstorm activity and squalls across the eastern Atlantic Ocean. This alignment favors yet another major winter storm (wind, rain, snow and cold) for much of the European subcontinent over the next week or so.
Yet another massive winter-type storm is affecting western Europe, succeeding a previous disturbance and yielding to a third gyre forming over the eastern Atlantic Ocean. Note the contrast of the cold, stormy European Union with very calm Africa, dominated by a vast heat ridge complex. The ITCZ is very dormant save for a patch of thunderstorms over the equatorial regions between the Cape Verde Islands and the Lesser Antilles.
A broad area of convection stretching from the Arabian Sea through Malaysia and Sumatra is associated with a flaring of the Madden-Julian Oscillation. The MJO signal is still retrogressing, and is not contributing to potential amplification of the polar westerlies over North America (which would seem to favor a warmer direction for the 11 – 15 day period over most of the U.S.). Arctic westerlies are clearly evident over Siberia; most of southern Asia is warm and dry.
SHORT RANGE OUTLOOK
(Through The Next 72 Hours)
Cold, Stormy Weather In The West….
Plymouth State University Weather Server
The spectacular storm complex that covers most of western North America and the eastern Pacific Ocean is triggering two notable effects on weather in the West. One is the drop in temperature, with the mA gyre channeling a southward flow from Alaska that ultimately should drive the nocturnal freeze line as far south as interior S CA. The other is supplying locally heavy precipitation, both in convective (thunderstorms) and stratiform (heavy rain and snow) types. This moisture is a good thing for the snowpack of the Intermountain Region, which has been on a downward spiral in recent years. Thus, the ski industry, agriculture, and urban water budgets should benefit from the three to four days worth of disturbed weather across the western third of the nation. Keep in mind, however, that the brutal cold regime will greatly impact Montana and Wyoming in the near term, giving a hard jolt of reality to locations that have not see such harsh cold in a number of years.
But Generally Mild And Calm From The Great Plains To The Eastern Seaboard
Ahead of the broad trough and storminess, most of the eastern half of the U.S. will be under a warming southwesterly flow regime. Note the heat ridge over Florida, a common synoptic signal during well-formed La Nina episodes. Despite the presence of strong -EPO and -NAO ridging, this subtropical high is forming a defense of sorts against an onrushing cA air mass. Not until Thanksgiving Day will the Arctic values break down the strong positive height anomaly. Until then, readings from the lower/middle Great Plains into ON and NY will continue well above normal, with rain and thunderstorms the most likely precipitation types.
MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)
Many Questions About Storm Potential….
Plymouth State University Weather Server
Plymouth State University Weather Server
You may have noticed that after a period of good agreement concerning the evolution of the storm track and precipitation shields, the numerical models are now in a chaotic phase. It still appears likely (to me at least) that cyclogenesis in southern Colorado will get underway late Tuesday, with a track toward the lower Great Lakes. So unlike what the European scheme was showing on Wednesday and Thursday, snow and wind threats may be more scattershot, with thunderstorms and rain being the main concern in parts of the Mid-South and Ohio Valley between November 23 and 26.
There are a number of computer models showing what appears to be another Colorado/Trinidad ‘A’ cyclone taking shape around November 29. The combination of a modest +PNA signature and a still sharply defined -NAO ridge complex across the Davis Strait and Baffin island (which the operational GFS version has had major trouble forecasting) may work with the lack of ridging in the Southeast to permit a colder, more southward reformation of the storm track. If so, then cities such as Chicago IL and Cleveland OH could receive important snowfall amounts just a few days after Thanksgiving.
….And Even More About Cold Intrusions!
I am being cautious with the very rapid cold to warm to cold sequence shown by the operational models during the medium range. For one, the presence of the -NAO ridge complex (which tends to retrogress into -AO territory by the 11 – 15 day time frame) would argue against fast alterations in the cold air presence over the northern half of the U.S. Secondly, the Colorado/Trinidad storm indicated by the ECMWF and GFS versions may be fairly deep and of the ‘B’ track scenario, which of course implies a colder solution along and above 35 N Latitude. In any event, a warm turn after November 27 is likely only for Texas and the Deep South. The Midwest and Northeast, it would seem, may still have to deal with negative thermal anomalies through much of the first week of December.
EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)
Will The -NAO Signal Stick Around? Or Is It “Pac Jet” Influence That Wins The Day?
Penn State University E-Wall
When you examine the longer term ensemble packages of the main weather forecast schemes, you are struck by three common points of interest. One, the strong, but somewhat retrogressive -NAO signature. Secondly, the basic semizonal nature of the polar westerlies, with no evidence of a block in the -EPO or +PNA positions. Third, the tendency of these model variants to display what appears to be two intense storms taking a track from CO or NM into the Mid-Atlantic region. Studies have shown that in order for a widespread cold spell to develop across the U.S., at least two blocking ridges must form at high latitudes. Last winter, you may recall, there were often THREE such signals (+PNA, -AO, -NAO) which doomed forecasts calling for a mild DJF period due to the strong El Nino episode. So with one large positive height anomaly, the jet stream will be suppressed somewhat but not for long periods.
The picture that emerges here is one where temperatures are coldest over the northern half of the U.S. (ironically while a good portion of Canada is warmer than average due to the ridge complex in Nunavut AR). If the storm track does move from CO to off of the Delmarva Peninsula (or in the ‘A’ scenario into N NY), then the southern tier of the nation will see stronger periods of warming than their counterparts up north. Should two storms traverse the country in the semizonal flow (note the disturbance son the water vapor imagery over the Pacific Ocean moving in a strong band of westerlies aloft), then snow and perhaps ice will be expanding in coverage across much of the Midwest and interior Northeast by December 6.
Drought, Snow Cover And Vulcanism Will Be Important Players In The Winter Forecast
NOAA/University Of Nebraska/USDA
There are three factors that all need to consider for how the winter forecast will turn out. One is snowpack, which has been steadily expanding into the West and in Canada, but not at all in the Great Lakes and Northeast. Another is drought, which has been a constant across the entire Old South and the Corn Belt. The third item on the weather agenda is the threat for a major volcanic event in the equatorial regions, with Mount Merapi in Java (Indonesia) still the leading candidate for a stratospheric venting explosion (that is tons of ash and particulate matter being forced above the tropopause at a low latitude).
U.S. locations will have a colder winter IF the snow cover shield expands to 40 N Latitude by December 15. Remember that snow is a refrigerant, and under high pressure (think clear skies and lack of wind) nocturnal lows will drop farther and faster if a snow field is deep. Plus, passage of cA regimes over the Great Lakes should have the effect of bringing the snow/ice line further south and east, in some cases as far as Appalachia.
If the wide area of dry soil continues through mid-December, it is a sign that a classic Florida/Georgia heat ridge, associated with La Nina, is the key feature for the winter. High 500MB heights in the Southeast and Gulf Coast invariably spell a warmer winter, since a strong storm cracking the anticyclone would invite higher dewpoints and more rain into Dixie and the Corn Belt. That moisture fetch, in turn, would allow for higher synoptic snowfall totals in the Midwest and Northeast, with “Miller B” redevelopment scenarios off of the Virginia Capes more commonplace.
Lastly, the potential for a volcanic eruption is sort of a “wild card” that can turn some winters with a mild character to much colder seasons filled with ice and snow. Evidence 1992-93 and 1993-94, affected by the blast of Mt. Pinatubo in the Philippines. That volcano shot a great deal of ash and particulate matter into the high atmosphere, creating a mechanism to cut down on sunlight. Global temperatures fell about .9 deg F on average, with the strongest effects reaching North America about 1 to 3 years later. Should a huge vulcan event occur in the tropics (below 25 Latitude), the potential for alterations in the current winter may increase. Mount Merapi in Indonesia of course, remains the most likely candidate for aerosol injection into the stratosphere. It would probably take a vertical ash and gas plume of 80 km for changes in atmospheric thermal values to occur.
Prepared by Meteorologist LARRY COSGROVE on
Saturday, November 20, 2010 at 5:00 P.M. CT
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 2010 by Larry Cosgrove
All rights reserved.
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