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7/18/13
The miserably hot and humid weather that has plagued Southeast Lower Michigan will come to a climatic end over the weekend as an active cold front pushes its way through the region Friday night into the first half of Saturday. While Saturday will cool off some with lower humidity, the full effects of the cooler air mass won't be felt until Saturday night into Sunday.
In the meantime until the front makes its way through; strong to severe thunderstorms are possible on Friday with the best chance of organized severe weather later Friday afternoon into Friday night. From the Storm Prediction Center /SPC/ this Thursday morning;
"NUMEROUS EPISODES OF STRONG TO SEVERE STORMS SEEM POSSIBLE IN THE
GREAT LAKES THROUGH THE DAY WITH THE GREATEST SEVERE POTENTIAL
COMING IN THE FORM OF DAMAGING WINDS AROUND THE TIME OF MAXIMUM
DIURNAL HEATING. IN ADDITION TO LOCALLY STRONG INSTABILITY...
STRENGTHENING UNIDIRECTIONAL VERTICAL SHEAR SHOULD CONTRIBUTE TO
RELATIVELY FAST-MOVING AND LONGER-LIVED LINEAR SEGMENTS WITH SOME
LEWPS AND BOWS RESULTING IN LOCALLY GREATER POTENTIAL FOR DAMAGING
WINDS AND PERHAPS A BRIEF TORNADO OR TWO."
And part of this afternoon's script from SPC;
A SHARPLY DEFINED COLD FRONT WILL ADVANCE SEWD ACROSS WI AND THE UPPER
PENINSULA OF MI BEING POSITIONED FROM LOWER MI WSWWD ACROSS FAR NRN IL
INTO IA BY LATE AFTERNOON. THE MODELS SUGGEST THAT THUNDERSTORMS WILL
INITIATE ALONG THE FRONT BY EARLY TO MID AFTERNOON AS SFC TEMPS WARM AND
MODERATE TO STRONG INSTABILITY DEVELOPS. THE INSTABILITY ALONG WITH LARGE-
SCALE ASCENT ASSOCIATED WITH THE UPPER-LEVEL TROUGH AND MODERATE TO STRONG
DEEP LAYER SHEAR SHOULD BE FAVORABLE FOR SEVERE WEATHER ACROSS MUCH OF THE
GREAT LAKES REGION FRIDAY AFTERNOON AND EVENING.
FORECAST SOUNDINGS AT DETROIT AND CHICAGO AT 00Z/SAT SHOW MLCAPE
VALUES FROM 2000 TO 3000 J/KG WITH 0-3 KM LAPSE RATES OF 8.0 TO 8.5
C/KM. THIS COMBINED WITH UNIDIRECTIONAL WIND PROFILES SHOULD BE
FAVORABLE FOR A WIND DAMAGE THREAT WITH MULTICELL LINE SEGMENTS THAT
ORGANIZE ALONG THE FRONT. AS STORM COVERAGE INCREASES DURING THE
LATE AFTERNOON...A NEARLY CONTINUOUS LINE COULD ORGANIZE AND MOVE
SWD ACROSS SRN LOWER MI EXTENDING WSWWD INTO NRN IND AND NRN IL
WHERE AN ENHANCED WIND DAMAGE THREAT MAY DEVELOP. DUE TO THE
MODERATE TO STRONG INSTABILITY...ISOLATED LARGE HAIL WILL ALSO BE
POSSIBLE WITH THE MORE INTENSE ROTATING CELLS EMBEDDED IN THE LINE.
ALTHOUGH THE LOW-LEVEL WIND PROFILES ARE FORECAST TO BE
UNIDIRECTIONAL...SUBSTANTIAL SPEED SHEAR IN THE BOUNDARY LAYER IS
EVIDENT ON FORECAST SOUNDINGS. THIS MAY ALLOW FOR A TORNADO THREAT
TO DEVELOP ESPECIALLY IF SUPERCELLS STRUCTURES CAN DEVELOP WITHIN OR
AHEAD OF THE LINE.
It would not surprise me if SPC upgraded some of the region under the slight risk to moderate risk at a later time given the severe weather parameters coming together Friday into Friday night.
And, checking out some of those parameters;
As of 5pm Friday, the projected maps;
CAPE on the map (basically the energy or "dynamite" in the air that is projected available for these storms).
In meteorology, convective available potential energy (CAPE),[1] sometimes, simply, available potential energy (APE), is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positive buoyancy of an air parcel and is an indicator of atmospheric instability, which makes it very valuable in predicting severe weather.
Here we are destined for 2500-3000 J/kg (measured in joules per kilogram) at maximum heating, certainly sufficient enough when combined with the other factors below for severe weather.
Resulted Lifted Index (measure of the unstable air) at maximum heating on Friday along with the cold front axis;
The lifted index (LI) is the temperature difference between an air parcel lifted adiabatically Tp(p) and the temperature of the environment Te(p) at a given pressure height in the troposphere (lowest layer where most weather occurs) of the atmosphere, usually 500 hPa (mb). When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable.
LI is generally scaled as follows:
- LI 6 or Greater, Very Stable Conditions
- LI Between 1 and 6 : Stable Conditions, Thunderstorms Not Likely
- LI Between 0 and -2 : Slightly Unstable, Thunderstorms Possible, With Lifting Mechanism (i.e., cold front, daytime heating, ...)
- LI Between -2 and -6 : Unstable, Thunderstorms Likely, Some Severe With Lifting Mechanism
- LI Less Than -6: Very Unstable, Severe Thunderstorms Likely With Lifting Mechanism
No question about the instability measured and available (ave of -7 > -9);
What about the shear available for the storms?
Bulk Shear
Bulk Shear - The bulk wind difference over a layer, calculated by vector subtraction. Bulk shear through 0-6 km AGL layer discriminates strongly between supercell and nonsupercell thunderstorm environments. The transition from nonsupercell to supercell thunderstorms occurs as the 0-6 km bulk wind difference increases from roughly 25 kt to 40 kt, with larger values favoring supercells. Research also suggests that increasing values in the 0-6km layer correlate with increasing tornado potential.
In the map below we look at the Bulk Shear from the surface to 500 MB (18,000 ft);
SFC-500MB Bulk Shear
Represents the boundary layer through 500MB above the ground, the shear vectors denote the change in wind throughout this height. Thunderstorms become more organized and long lived as vertical shear increases. Supercells are commonly associated with vertical shear values (Bulk shear values) of 35-40KT or better at this height.
While we have strong bulk shear in north and central Michigan, it is just beginning to nose into Southern Michigan - so it's CORE placement is a bit far north for Southeast Lower Michigan but in time, brushes the region. It will affect the storms developing to our northwest (most likely the squall line that is generated that will push through the region in the evening).
Upper Jet: (at 34,000 ft);
The most important feature to the meteorologist at the 250mb pressure level is the winds. The winds are important as its the typical location of the jet stream. The jet stream at 250 MB is located about 34,000 feet /33,000-35,000ft/. In this case, the right-rear quadrant of the wind core (which aides effectively in storm development) is a powerful jet core of 75-90 knots crossing northern and central Michigan late Friday afternoon and evening.
Finally;
Of course, this is all in projected model-land /GFS/ and has been, for a few days now and does bear watching for confirmation! If the atmosphere dynamics come together as projected, severe weather including a squall-line is likely!
Making weather fun while we all learn,
Bill Deedler - SEMI_WeatherHistorian
