Arcus Clouds, Relief from the Heat. Severe Weather, and 98L

What is an Arcus Cloud ? Have you ever heard or seen one ? You will find out shortly.

A stubborn dome of the high pressure that has been suppressing thunderstorms and is responsible for the dangerous July heat wave will pack a final punch of crazy hot temperatures and high humidity through late this week from the Plains to Mid-Atlantic.
The break in the extreme heat will come from a cold front that will erode away the northern portions of the heat ridge in the Middle Mississippi Valley and central Plains later today. The heat will be broken on Thursday for the Midwest and Ohio Valley, and in the Mid-Atlantic on Friday. Gone will be widespread 90s and triple-digits, replaced by seasonable mid-summer 80s and lower 90s. Even a few upper 70s will sneak into the Midwest and Middle Mississippi Valley as high pressure brings refreshing Canadian air as far south as Iowa, and from northern Illinois to southern Michigan.

The good news is temperature levels won`t be as dangerous on Thursday, but folks across the Interstate 95 corridor in the Mid-Atlantic will be facing triple-digit heat before a cold front inches it off the East Coast by this weekend. Heat Advisories and Excessive Heat Warnings are in place from central Kansas and most of Iowa and southern Wisconsin arcing all the way to southern Georgia today and Thursday. Additional heat-related advisories may be required for the Mid-Atlantic on Thursday.



As we all know, whenever the Hot, meets the Cold, something has got to give. The battle lines have been drawn once again as severe weather is, and will be occurring throughout the day. Severe thunderstorms are rolling along the "ring of fire," or where the river of air at high altitudes in the atmosphere known as the jet stream, direct disturbances around the Plains heat ridge. Severe thunderstorms are moving across Northern Minnesota this morning and will be a problem for the Great Lakes region through tonight, with additional thunderstorms building along a cold front from eastern Nebraska to southern Minnesota this afternoon. Destructive wind gusts are the biggest threat.



These thunderstorms have a history of producing quarter-to-golf ball size hail in Fargo, N.D., and a wind gust of 70 mph damaged homes, trees, and crops near Regent, N.D.

After powerful thunderstorms slammed into Chicago and parts of the Upper Midwest and western Great Lakes on Tuesday, another round of damaging storms will rumble across many of the same areas today and tonight. Chicago received a lot of damage yesterday morning as a Derecho style line of storms moved through the city.




Today's Convective Outlook from the SPC.



An atmospheric recipe of blazing hot and humid air and a cold front slicing into the region will provide the spark for severe thunderstorms to ignite. Temperatures from southern Iowa to central Illinois will surge into the 100s, while areas just farther north including Chicago will soar into the mid-90s. The storms that develop this afternoon will be capable of damaging straight-line winds, large hail, blinding downpours and dangerous lightning.



While the overall threat for tornadoes is low, a twister or two touching down and causing damage cannot be ruled out.

An area of Low Pressure was classified yesterday by the NAVY as 98L. This area of concern is located about 800 miles East - Northeast of Bermuda and is moving around 20mph to the Northeast. This area is not a threat at this time.



Another area to keep an eye out for the the wave that just came off the coast of Africa. While the environment is not currently favorable for this wave to develop, conditions might improve as this wave approaches the Antilles.



Now, back to the Arcus thing.

An arcus cloud is a low, horizontal cloud formation. We normally call them shelf clouds. Roll clouds and shelf clouds are the two types of arcus clouds. A shelf cloud is usually associated with the leading edge of thunderstorm outflow; roll clouds are usually formed by outflows of cold air from sea breezes or cold fronts in the absence of thunderstorms.

What makes me bring this up is that yesterday, the people of Richmond were freaking out after this monster cloud invaded the city.



Shelf clouds often form at the leading edge of a gust front or outflow boundary from a thunderstorm, or strong winds flowing down and outward from a storm. The outer part of a shelf cloud is often smoother with a notable rising motion exhibited by a tiered look (hence, the name shelf cloud!). Underneath, a turbulent, unsettled appearance is often the case. A shelf cloud should be seen as a harbinger of strong winds.

Got to watch this video. Warning ,one or two bad words. This dude cracks me up. Makes weather interesting.

Great write up. Thanks E.

LMAO e...that guy was funny...but I sure am glad they uploaded that video....very cool cloud!!

What a great blog, today!! Thanks for all your work! I like the other video you put in here, also.....guess we might be watching for storm chasers to set up today.....

Another cool shelf cloud moving over Richmond a couple weeks ago.

Hey e....kinda curious as to his model selection in the video....any thoughts?

sangria wrote:Hey e....kinda curious as to his model selection in the video....any thoughts?

He references the GFS and Euro in his video, not unless you are talking about the dude from Richmond and he is using the BTAN models. Bob's This Ain't Normal model. That is strictly a line of site model

But back to the tropical update video, I agree based on the current conditions. If my memory serves me right, the GFS developed this thing for about 6 to 8 runs then dropped it yesterday. But it is still a wave and will have to be watched. If the trough moves out of the way the chances of something to develop are much greater

Thanks e.....

Now, for my forecast.....Hot Hot Hot!!!!!!!

Great blog e! Man you are putting great effort into this and it shows. Kudos!!!

MESOSCALE DISCUSSION 1583
NWS STORM PREDICTION CENTER NORMAN OK
1239 PM CDT WED JUL 25 2012

AREAS AFFECTED...MUCH OF GA...ERN AL...NRN FL PENINSULA...CNTRL/ERN
FL PANHANDLE

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 251739Z - 252015Z

PROBABILITY OF WATCH ISSUANCE...5 PERCENT

SUMMARY...PULSE-TYPE STORMS MAY POSE A LOCALIZED SVR THREAT THROUGH
THE AFTERNOON. GIVEN THE DISORGANIZED NATURE OF THE CONVECTION...AND
THE SPORADIC NATURE OF ANY SVR STORMS...A WW IS NOT NEEDED.

DISCUSSION...AMIDST A MOIST AIR MASS CHARACTERIZED BY PW OF
1.75-2.00 INCHES PER GPS DATA...STRONG DIURNAL HEATING WILL CONTINUE
TO SUPPORT DESTABILIZATION. MODIFIED 12Z OBSERVED SOUNDINGS SUGGEST
THAT MLCAPE VALUES OF 2500-3500 J/KG WILL BE COMMON ACROSS THE AREA
THROUGH THE MID/LATE AFTERNOON.

WITH LITTLE/NO INHIBITION...CONVECTION WILL CONTINUE TO
DEVELOP/INTENSIFY FROM A VARIETY OF SOURCES:
/1/ DEVELOPING SEA BREEZE CONVERGENCE ZONES OVER INLAND AREAS
ADJACENT TO THE GULF/ATLANTIC WATERS...
/2/ A FIELD OF ENHANCED BOUNDARY-LAYER CUMULUS EXTENDING FROM FAR
SERN AL INTO SWRN/S-CNTRL GA AND PARTS OF THE FL PANHANDLE -- I.E.
ACROSS THE WARM SIDE OF A DIFFERENTIAL HEATING ZONE PERIPHERAL TO A
HIGH-CLOUD SHIELD ACCOMPANYING CLUSTERS OF CONVECTION OVER THE
OFFSHORE WATERS OF THE NERN GULF...
/3/ NEAR AND JUST SW OF A STALLED FRONT ARCING FROM SWRN NC INTO
NERN GA AND FAR SRN SC...AND
/4/ CUMULUS-CONVERGENCE LINES WITHIN THE UNSTABLE SECTOR SW OF THE
FRONT.

GIVEN THE AFOREMENTIONED DEGREE OF BUOYANCY...VIGOROUS
UPDRAFTS/DOWNDRAFTS ARE ANTICIPATED WITH CONVECTION THIS AFTERNOON.
SPORADIC INSTANCES OF DMGG WIND GUSTS AND MARGINALLY SVR HAIL WILL
BE POSSIBLE. HOWEVER...WITH AOB 15 KT OF FLOW EVIDENT BELOW THE 4-KM
AGL LEVEL PER AREA VWP DATA...WEAK LOW/MID-LEVEL FLOW WILL FOSTER A
PULSE/DISORGANIZED CONVECTIVE MODE. ANY INSTANCES OF SVR STORMS
SHOULD REMAIN ISOLATED/SPORADIC.

Tomorrow is looking like it is going to be a NASTY day in the Northeast.

Fortunately, it was a quiet day, today........

Heck...we go from no mesoscale discussions to five in about two hours........guess this evening might be busy......

http://www.spc.noaa.gov/products/md/

Gonna throw a little of the tropics in here....looks like the wave that just came off of the coast of Africa, has a little spin to it......



Most recent Oscat....

URGENT - IMMEDIATE BROADCAST REQUESTED
SEVERE THUNDERSTORM WATCH NUMBER 519
NWS STORM PREDICTION CENTER NORMAN OK
515 PM CDT WED JUL 25 2012

THE NWS STORM PREDICTION CENTER HAS ISSUED A
SEVERE THUNDERSTORM WATCH FOR PORTIONS OF

NORTHERN IOWA
SOUTHERN MINNESOTA
SOUTHWEST WISCONSIN

EFFECTIVE THIS WEDNESDAY AFTERNOON AND THURSDAY MORNING FROM 515
PM UNTIL 100 AM CDT.

HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70
MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.

THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 85
STATUTE MILES EAST AND WEST OF A LINE FROM 40 MILES WEST
SOUTHWEST OF DENISON IOWA TO 30 MILES NORTH NORTHWEST OF LA
CROSSE WISCONSIN. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE
ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU9).

REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE
FAVORABLE FOR SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
AND POSSIBLE WARNINGS. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY
DO PRODUCE TORNADOES.

OTHER WATCH INFORMATION...CONTINUE...WW 518...

DISCUSSION...TSTMS ARE EXPECTED TO INCREASE IN COVERAGE ALONG THE
COLD FRONT THIS EVENING AS REGION IS GLANCED BY A WEAK MIDLEVEL
IMPULSE MOVING INTO CNTRL/SRN MN. THERMODYNAMIC ENVIRONMENT IS
CHARACTERIZED BY A HOT...MOIST BOUNDARY LAYER WITH MLCAPE OF
2500-3500 J/KG. WHEN COUPLED WITH MODEST DEEP...WLY SHEAR...SETUP
WILL FAVOR MULTICELLS AND LINE SEGMENTS CAPABLE OF DAMAGING WINDS
AND LARGE HAIL.

AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT
TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60
KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500. MEAN STORM
MOTION VECTOR 28020.

URGENT - IMMEDIATE BROADCAST REQUESTED
SEVERE THUNDERSTORM WATCH NUMBER 520
NWS STORM PREDICTION CENTER NORMAN OK
640 PM CDT WED JUL 25 2012

THE NWS STORM PREDICTION CENTER HAS ISSUED A
SEVERE THUNDERSTORM WATCH FOR PORTIONS OF

WESTERN LOWER MICHIGAN
CENTRAL AND EAST-CENTRAL WISCONSIN
LAKE MICHIGAN

EFFECTIVE THIS WEDNESDAY NIGHT AND THURSDAY MORNING FROM 640 PM
UNTIL 200 AM CDT.

HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70
MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.

THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 55
STATUTE MILES NORTH AND SOUTH OF A LINE FROM 20 MILES NORTH OF
CAMP DOUGLAS WISCONSIN TO 45 MILES EAST OF ROBEN HOOD MICHIGAN.
FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH
OUTLINE UPDATE (WOUS64 KWNS WOU0).

REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE
FAVORABLE FOR SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
AND POSSIBLE WARNINGS. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY
DO PRODUCE TORNADOES.

OTHER WATCH INFORMATION...CONTINUE...WW 518...WW 519...

DISCUSSION...RECENT TSTM INITIATION HAS OCCURRED JUST W OF GRB INVOF
SURFACE LOW AND COLD FRONT WHERE LOW-LEVEL CONVERGENCE IS BEING
ENHANCED. STRENGTHENING SWLY LLJ THIS EVENING SHOULD BOOST
CONVERGENCE ALONG THE COLD FRONT...FOSTERING AN INCREASE IN STORMS
OVER E-CNTRL WI WITHIN AN ENVIRONMENT CHARACTERIZED BY MLCAPE OF
2000-3000 J/KG AND 40-50 KT OF DEEP-LAYER SHEAR. INITIAL STORM MODE
MAY BE SUPERCELLULAR WITH A RISK FOR LARGE HAIL AND POSSIBLY A
TORNADO. UPSCALE GROWTH INTO A BOW ECHO IS ANTICIPATED LATER ACROSS
LOWER MI WHERE CORRIDORS OF WIND DAMAGE WILL BE POSSIBLE.

SPC Upgraded the Slight Risk to Moderate Risk for tonight.

Thanks for the updates, e. Gonna be a wild night for some folks!