Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 1:43 PM GMT on May 31, 2012
A complex synoptic setup, more reminiscent of mid Fall than early June, will advance eastward towards the Atlantic coast on Friday leaving a path of high-impact weather across the region.
A 1004hPa surface low will track northeast through central Ohio by Friday morning along with a trailing cold front that will become the catalyst for convective development. Meanwhile a warm front will begin lifting north over the Middle Atlantic. Current NAM/ECMWF guidance suggests it reaches a region towards southern Pennsylvania. This warm sector will be the focus for the strongest thunderstorms. The exact position of the northern extent of the warm front is still uncertain. The easterly flow off the Atlantic looks to be relatively weak after analyzing the HIRES NMM. This may allow the warm front to surge a bit farther north than progged. Therefore the warm sector may approach the I-80 corridor. The negatively titled trough will promote a strong southerly low level jet along and just to the east of the approaching cold front.
Relatively little to no dying convection ahead of the front will prevent widespread cloudiness in the morning outside some low level stratus from the onshore flow. And as the warm front lifts north, the low level clouds will erode giving way to partly cloudy skies. The amount of sunshine is also uncertain, but given the extensive dynamics of the storm system, it is unnecessary for thermodynamics to reach high values to fuel convection.
By mid morning most areas from northern Virginia up through central Maryland to south-central Pennsylvania will begin destabilizing as SBCAPE values rise to upwards of 1500 j/kg. Current NAM indices also indicate lifted index values around -5C. The area of highest instability looks to be progged across northern Virginia into central Maryland where the greatest amount of sunshine is possible. But the strongest dynamics will be displaced a tad to the north along the warm front axis likely draped across an area between I-80 and I-76. Helicity values will approach an impressive 300 m^2 s^-2. Helicity is a good measure of the amount of rotation/spin in the atmosphere. Any values above 100 m^2 s^-2 are pretty impressive for the east coast. The combined helicity and instability index is the EHI. The index is a good measure for the risk of potential tornadic development. EHI values will be nearing 1-3.5 for parts of northern Maryland into southcentral Pennsylvania. Total 0-6km shear values will also be a potent 40-50 knots especially across southern Pennsylvania and northern Maryland.
A few soundings from the York, PA - Baltimore, MD corridor indicate impressive curved hodographs indicate severe weather development possible. PWATs will rise to near (+1)-(+2)SD and around 1.5-1.7in. This will also promote heavy rain. While the thunderstorms will likely be cellular ahead of the main linear band, they will be moving at a relatively quick pace given the strength of the low level jet. But the threat of training thunderstorms is possible along the main linear band, and this will promote a minor flash flooding threat as rainfall amounts exceed 2in in localized areas. I am not very concerned about the flash flooding threat in general.
As noted above by the impressive dynamics and thermodynamic fields, an enhanced risk for damaging winds and tornadoes are likely for a confined area from northern Virginia up through Washington DC and central Maryland north to the Lower Susquehanna Valley. This corridor will have the highest risk and likely will be under a SPC moderate risk at some point in the event. The proximity of the warm front and impressive helicity/shear will allow for thunderstorms to easily develop at least low level spin.
As for the evolution of the thunderstorms, I would expect a linear band of severe thunderstorms to begin to form across West Virginia up through western Pennsylvania by midday Friday. As it moves into an area of higher SBCAPE, development will increase. Ahead of the squall line, supercellular development is possible especially across northern Maryland where the highest instability axis will exist in correspondance with the best dynamics. Isolated tornadoes are possible. The main line will focus in with several smaller bow echoes, but localized tornado spin-ups are also possible. Weaker MUCAPE around 500-1000 j/kg will limit their eastward March once past the Reading-Lancaster corridor. Lightning will begin to decline as they become more elevated in nature and weaken greatly towards the New Jersey/Pennsylvania border south to the Delmarva.
A few concerns remain... 1) The extent of destabilization from the clearing of the low level cloud deck. Although this looks pretty widespread especially areas south of the warm front. 2) The exact position of the warm front is most in question. Timing will be critical. At this time I expect the warm sector to reach as far north as the Lower Susquehanna Valley. 3) Recent HIRES guidance suggests a weak CAP especially towards eastern Maryland and northern Virginia, promoting more weakly organized severe clusters, but again given the amount of shear and steep lapse rates, I do not expect this to be a major problem.
This is one of the more impressive severe weather setups for the Middle Atlantic that I have seen progged in a few years especially for the tornado risk. I would expect several reports tomorrow. For areas in south-central Pennsylvania down through central Maryland, it will remain critical to stay tuned to the latest statements from your local CWA National Weather Service offices. Knowing how fragile severe weather setups are for the Middle Atlantic, I would take caution in the forecast. There are many factors in place that will be difficult to pin-point. Stay tuned!
Below are a few maps of interest...
5/31/12 6z NAM 0-1km EHI
5/31/12 6z NAM Significant Tornado Parameter
5/31/12 6z GFS Bulk Wind Shear 0-6km
5/31/12 0z NMM Simulated Radar
"Severe Weather Links"
-Atmospheric Soundings Skewt T charts...Link
-SPC Mesoscale Analysis Pages...Link
-Severe Weather Model Forecast indices...Link
-Severe Weather Parameter Definitions...Link
I believe now is the time to review some typical severe weather indices, warnings, and forecasting techniques. Let me start with tornadoes. Here is a blog I wrote back in March of 2008 that describes some easy tornado forecasting techniques...Link Now below are some common severe weather terms and their definitions.
Here are some definitions of warnings that I may mention during some severe weather outbreaks all courtesy of NOAA...Link
Severe Thunderstorm or Tornado Watch: Severe thunderstorms with large hail, damaging winds, and/or tornadoes are possible, but the exact time and location of storm development is still uncertain. A watch means be prepared for storms
Severe Thunderstorm Warning: A severe thunderstorm is imminent or occurring; it is either detected by weather radar or reported by storm spotters. A severe thunderstorm is one that produces winds 58 mph or stronger and/or hail 3/4 inch in diameter or larger. A warning means to take shelter.
Tornado Warning: A tornado is imminent or occurring; it is either detected by weather radar or reported by storm spotters. A warning means to take shelter.
Severe Weather Indices/Parameters all courtesy of NOAA...Link
Lapse rate: Refers to the rate of temperature change with height in the atmosphere. A steep lapse rate is one in which the environmental temperature decreases rapidly with height. The steeper the environmental lapse rate, the more potentially unstable is the atmosphere
LI Index: The LI is a commonly utilized measure of stability which measures the difference between a lifted parcel's temperature at 500 mb and the environmental temperature at 500 mb. It incorporates moisture and lapse rate (static stability) into one number, which is less vulnerable to observations at individual pressure levels. However, LI values do depend on the level from which a parcel is lifted, and rally cannot account for details in th environmental temperature curve above the LCL and below 500 mb. LI was originally intended to utilize average moisture and temperature properties within the planetary boundary layer. The Best LI represents the lowest (most unstable) LI computed from a series of levels from the surface to about 850 mb. This index is most useful during cases when shallow cool air exists north of a frontal boundary resulting in surface conditions and boundary layer-based LI values that are relatively stable. However, the airmass at the top of the inversion, from which lifting may occur, is potentially unstable. An example of this would be elevated ("overrunning") convection (possibly a nocturnal MCS).
Severe Weather Threat Index: The SWEAT Index evaluates the potential for severe weather by combining several parameters into one index. These parameters include low-level moisture (850 mb dewpoint), instability (Total Totals Index), lower and middle-level (850 and 500 mb) wind speeds, and warm air advection (veering between 850 and 500 mb). Therefore, an attempt is made to incorporate kinematic and thermodynamic information into one index. As such, the SWEAT index should be utilized to assess severe weather potential, not ordinary thunderstorm potential. These are guidance values developed by the U.S. Air Force. Severe storms may still be possible for SWEAT values of 250-300 if strong lifting is present. In addition, tornadoes may occur with SWEAT values below 400, especially if convective cell and boundary interactions increase the local shear which would not be resolved in this index. The SWEAT value can increase significantly during the day, so low values based on 1200 UTC data may be unrepresentative if substantial changes in moisture, stability, and/or wind shear occur during the day. Finally, as with all indices, the SWEAT only indicates the potential for convection. There must still be sufficient forcing for upward motion to release the instability before thunderstorms can develop.
Convective Available Potential Energy: CAPE assumes Parcel Theory, in that 1) a rising parcel exhibits no environmental entrainment, 2) the parcel rises (moist) adiabatically, 3) all precipitation falls out of the parcel (no water loading), and 4) the parcel pressure is equal to the environmental pressure at each level. Parcel Theory can have significant errors, especially for large parcel displacements, at cloud edges, and for significant water loading. However, the method often works quite well in the undiluted core of a thunderstorm updraft. CAPE represents the amount of buoyant energy available to accelerate a parcel vertically, or the amount of work a parcel does on the environment. CAPE is the positive area on a sounding between the parcel's assumed ascent along a moist adiabat and the environmental temperature curve from the level of free convection (LFC) to the equilibrium level (EL). The greater the temperature difference between the warmer parcel and the cooler environment, the greater the CAPE and updraft acceleration to produce strong convection.
Convective Inhibition: CIN represents the amount of negative buoyant energy available to inhibit or suppress upward vertical acceleration, or the amount of work the environment must do on the parcel to raise the parcel to its LFC. CIN basically is the opposite of CAPE, and represents the negative energy area (B-) on the sounding where the parcel temperature is cooler than that of the environment. The smaller (larger) the CIN is, the weaker (stronger) must be the amount of synoptic and especially mesoscale forced lift to bring the parcel to its LFC. High CIN values in the presence of little or no lift can cap or suppress convective development, despite possibly high CAPE values. Remember, CAPE is the "available potential" energy. That energy must be released to become "kinetic" energy to produce thunderstorms.
Helicity: Storm-relative (S-R) helicity (Hs-r) is an estimate of a thunderstorm's potential to acquire a rotating updraft given an environmental vertical wind shear profile, assuming thunderstorms are able to develop. It integrates the effects of S-R winds and the horizontal vorticity (generated by vertical shear of the horizontal wind) within the inflow layer of a storm. A S-R wind is the wind that a thunderstorm actually "feels" as the storm moves through the environment. It is different from a true ground-relative (G-R) wind, except for a stationary storm whereby a S-R and G-R wind are equivalent. S-R helicity is proportional to the area "swept out" by the S-R wind vectors between two levels on a hodograph.
Energy-Helocity Index: CAPE and storm-relative (S-R) helicity (Hs-r) are both very important in the formation of a strongly rotating convective updraft. CAPE represents the amount of buoyant energy available, while S-R helicity incorporates the effects of environmental vertical wind shear and storm motion on thunderstorm type and evolution. An intense rotating updraft can form with relatively weak CAPE if the vertical wind shear and storm-relative inflow are strong. On the other hand, relatively low S-R helicity usually can be compensated by high instability to produce a rotating updraft. The EHI attempts to combine CAPE and S-R helicity into one index to assess the potential for supercell and mesocyclone development. High EHI values represent an environment possessing high CAPE and/or high S-R helicity.
I hope these definitions help to make it a little easier to understand the terminology that you may hear during times of severe weather.
Lower Susquehanna Valley Doppler...
(Courtesy of WGAL)
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"Linglestown, PA 2012 statistics"
(Severe Weather Stats...)
Severe Thunderstorm Watches- 3
Severe Thunderstorm Warnings- 4
Tornado Watches- 1
Tornado Warnings- 0
Total Thunderstorms- 12
Flood Watches- 0
Flood Warnings- 0
Flash Flood Watch- 1
Flash Flood Warnings- 0
January precipitation- 2.82"
February precipitation- 1.90"
March precipitation- 1.41"
April precipitation- 1.74"
May precipitation- 7.47"
June precipitation- 1.06"
Yearly Precipitation- 16.40"
Heat Advisories- 0
Excessive Heat Warnings- 0
90F days- 2
Highest Temperature- 90F on 5/28 and 5/29
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Light Snow Mist