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FCST: September 6, 2010 ~ Lots of shear but no moisture!!

TARGET: Cedar Rapids, IA by 20Z/6.

DISCUSSION:  A compact and potent late season shortwave (995 mb) is moving across the central plains this afternoon.  This storm system will provide the threat of severe thunderstorms across eastern IA, southern MN and western WI later this afternoon and evening. 

The storm has plenty of shear to work with, but the rounds of recent cold fronts have scoured out most of the moisture from the area.  Couple in the progressive nature of the trough and you get plenty of shear and no moisture!

A 45 to 55 kt low level jet will be working its way into the target area by 0Z.

Currently, warm air advection is ongoing across eastern KS and western MO but even wtih that dewpoints have only recovered into the upper 50’s to lower 60’s.  Forecasted dewpoints had been in the upper 60’s to lower 70’s.  I do not feel these dews are going to be realized over the aforementioned areas based on my current thinking.

CAPE is forecasted to be in the 1000 to 1500 J/kg range by this afternoon which is appreciable for severe weather. 

A stout EML is advecting into the region as well and will need the approaching shortwave to provide the needed forcing to oversome the capping inversion. I am anticipating some early, isolated supercells from Des Moines northeastward that will quickly congeal into an MCS and march eastward overnight creating a damaging wind threat.

The early storms will yield any tornado threat in the target area.   Helicity values are maximized in these areas and a strong tornado is possible, but not probable in this amateur forecasters opinion. ~ CS

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Tropical Storm Hermine ~ Gulf of Mexico

UPDATE September 8, 2010 :

Remnants of Tropical Storm Hermine to have an effect on the Kansas / Missouri area the next couple of days.  Here is a look at the forecasted track of the low and Days 1-2 QPF Forecasts overlayed in Google Earth:


Tropical Storm Hermine is approaching the Mexico/Texan coast and shows signs of gradual strengthening and will most likely be a Category One Hurricane when it makes landfall.  Currently, Tropical Storm Hermine is located at 24.1N and 96.5W, about 140 miles south.southeast of Brownsville, TX.  Maximum sustained winds are 60 mph with a movement to the north/northwest at 14 mph.  The minumum central pressure is 995mb (29.38 inches).

A hurricane watch is in effect from Rio San Fernando Mexico northward to Baffin Bay, TX.  A tropical storm warning is in effect from La Cruz Mexico to Port O’Connor, TX.


Here is the current forecasted track for Hermine the next few days:

Here is a look at the latest visible satellite imagery for Hermine.  Gradual strengthening is forecasted over the next 12 to 24 hours before it makes landfall over the Mexico/Texas border region. ~ CS

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Hurricane Earl ~ Category 4 Hurricane

A lot of folks have posted things on their websites and blogs on Hurricane Earl.  I cannot let this historic, near miss, Hurricane go by without a few comments.  Hurricane Earl, a Category 4, Hurricane on the Saffir-Simpson wind scale is having effects already on the east coast.  Damage has been reported in the leeward Islands as well as Puerto Rico, but they were spared the brunt of the Hurricane.

As of 7 p.m. CDT Earl was located at 27.2 N and 73.5W or about 565 miles south/southeast of Cape Hatteras, NC.  Maximum sustained winds are 135 mph with a movement to the North/Northwest (330 degrees) at 18 mph.  Hurricane force winds extend outward up to 90 miles and Tropical Storm Force winds extend outward 200 miles.  The minimum central pressure is 941 millibars.

Sea temperatures near and just ahead of Earl are warm enough to allow it to overcome some of the shear associated with the approaching trough.  However, an overall loss of intensity is expected as Earl approaches the eastern seaboard.  In addition, the forecast models are indicating Earl will take a more northerly track as it approaches the Carolinas.  This is good news for the east coast residences.  Here is a look at the SST’s Earl has to work with:

Hurricane Earl is still forecasted to approach the North Carolina coast during the day on Thursday and move over the outer banks through the night Thursday.  This is a major Hurricane approaching the US mainland.  Persons in these areas should be prepared now for Hurricane and/or Tropical Storm conditions the next 72 to 96 hours.

Image Courtesy of NASA Goddard Photo and Video Page on

The trough that brought the rain and thunderstorms overnight is, most likely, going to be the saving grace for the mid-Atlantic states.  (Visible in the above image)  The trough, along with increased shear, will interact with Earl and push it out to sea, allowing only a glancing blow to the eastern seaboard.  Albeit, damage may be significant but not near the damage that a landfalling major hurricane would cause.  Here is the NAM-WRF forecasted position as of 7 a.m. Friday morning September 3:

Currently, Hurricane/Tropical Storm Watches and Warnings are up all along the eastern seaboard.  Here is the latest watch/warning information from the NHC. ~ CS

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Tropical Storm Alex forecasted to be Cat 2 Hurricane

EDIT: 10 p.m. CDT  Hurricane Warnings are now posted for the Texas Coast from Baffin Bay to the mouth of the Rio Grande.  Hurricane conditions are expected within the next 36 hours.  Maximum sustained winds are 65 mph.  Currently, Alex is located 505 miles south of Brownsville, TX at 21.0N and 91.6W.  The minimum central pressure is 985mb and is moving north at 5 mph.  Alex will continue to strengthen overnight and is expected to be a Hurricane by Tuesday morning.


Tropical Storm Alex has moved into warmer waters in the western Carribean after leaving behind heavy rains over the Yucatan Pennisula.  The storm, as of 10 a.m. CDT, is located at  20.3N and 91.7W (about 535 miles SE of Brownsville, TX) and is moving to the NNW at 7 mph and is forecasted to turn more NW on Tuesday.  The minimum central pressure is 989mb.   

The National Hurricane Center is forecasting Alex to strengthen to Hurrican Strength overnight tonight into Tuesday.  Current progs are for Alex to reach Category 2 on the Saffir-Simpson scale and make landfall in northern Mexico and Southern Texas within the next 48 to 72 hours.  The exact track and landfall location for Alex is a bit uncertain at this time.  A Hurricane Watch has been hoisted for these areas. 

Here is the latest forecasted track for Alex and location of it making landfall.

Persons with interests in the affected area are urged to keep track of the latest bulletins from the National Hurricane Center and local Weather Forecast Offices.

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FCST: May 22, 2010 NE/SD

Initial Target: O’Neill, NE to Plankinton, SD (Based on a blend of the NAM-WRF, GFS, SREF and ECMWF)

Forecast Discussion: A trough is digging onto the west coast with severel shortwaves visible at the midlevels.  Several of these shortwaves will round the base of the trough and eject into the northern plains during the period.  One thing to note is the trough remains well west of the modest instability axis leaving any forcing to occur along the dryline Saturday afternoon and evening. 

Another concern is the EML forecasted to be strengthening over the target area throughout the day on Saturday.  Absent any large scale ascent this could be problematic for convection to initiate. However, the NAM is breaking precipitation out over south central SD and north central NE at 0Z/23, albeit it has been pushed further north from run to run.  Any storms that do form will quickly go severe and with a warm front in the area and the potential for outflow boundaries the tornado risk should be enhanced with the more robust supercells.

Another concern for this forecast period is the on-going thunderstorms over the high terrain of eastern Wyoming and the residual cloud cover. These parameters will need to be resolved with tomorrow mornings forecast and nowcasting throughout the morning on Saturday.

Areas of surface low pressure are forecasted to be over eastern Montana (992mb) and NE Colorado (1000mb) with a developing surface low (996mb) over central Nebraska Saturday afternoon and evening.  This developing surface low is forecasted to deepen by 0Z/23. 

A warm front will be draped southeastward from the developing central Nebraska low back across south central and southeastern Nebraska.  A dryline will be positioned along and south of the low pressure center from the central Dakota’s into Kansas.  These areas of low pressure and attendant boundaries will interact with a very broad warm sector and will provide the focus for severe thunderstorm development from north central Kansas into the Dakota’s Saturday afternoon and evening.

Warm air advection is currently on-going over the target area with deep gulf moisture being transported northward into the forecasted warm sector.  A very broad warm sector will develop from central Kansas all the way into the Dakota’s tomorrow afternoon and evening.   Surface temperatures are forecasted to be in the upper 70’s to low 80’s across the target area.  Here is a look at current dewpoints and forecasted dewpoints over the target area (0Z/22):

Surface based CAPE is forecasted in the 3000 to 3500 J/kg range over the target area by the NAM-WRF and GFS.  There will be a moderate to strong cap over the target area most of the day with 700mb temps in the 12C-14C range.  Howevver, the NAM-WRF and GFS show the cap begining to erode between 18Z/22 and 0Z/23 over the target area with 700mb temps dropping to the 9C range as well.

Modest shear values are also present with bulk shear in the 40 to 50 kt range with shear vectors being perpindicualr to the dryline/boundary.  SRH and EHI values are maximized across the target area at 0Z/23.  Low level shear, 0-1km, is also appreciable for an attendant tornado threat with large, looping hodographs in the target area.

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steve polley storm chaser