Friday Map Discussion: 30 March 2007

All attachments: http://www.atmos.albany.edu/student/heathera/mapdisc_03-30-07/

Hi Folks,

Friday map discussion for 30 Mar'07 focused on the environmental conditions that have contributed to repeated severe weather episodes over the last 7-10 days in the area from just east of the Rockies to

the Mississippi River Valley. The following images are attached in support of the discussion:

1) Mean and anomaly sea level pressure (SLP, hPa) over the US for 21-30 Mar'07.

2) As in 1) except for 300 hPa heights (dam).

3) As in 1) except for 200 hPa vector winds (m/s).

4) Pressure (hPa) on the dynamic tropopause (DT), 850-DT shear (kt) and 925-850 hPa layer-mean relative vorticity (contour interval 0.5 x 10-4 s-1) for 0000 UTC 27-29 Mar'07.

5) As in 4) except sea level pressure (hPa), 1000-500 hPa thickness (dam) and 250 hPa isotachs (kt; color bar).

Images in 1-3 were generated from www.cdc.noaa.gov while images in 4-5 were created from

www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.php

From 1) it is evident that the 10-day time mean and anomaly SLP favors an anomalous transport of

air from the Gulf of Mexico poleward toward the northern Plains and westward toward the front range of the Rockies.

From 2) it is apparent that an anomalously deep time-mean trough, close to being cutoff, was

situated over the Southwest. Southwesterly flow ahead of this trough/cutoff ensured the existence of a

deep layer of warm-air advection and veering winds from the Rockies eastward across the Plains.

From 3) it is clear that the 10-day mean vector winds are weak over the eastern Great Basin in the

core of the trough/cutoff and strong (~40 m/s) in the subtropical jet (STJ) core over northern Mexico and

extreme southwestern Texas. The poleward (left) exit region of this time-mean STJ is positioned over

extreme eastern New Mexico, central and northern Texas and western Oklahoma. Given the persistence of the 10-day time-mean flow in the last part of March, the position of the STJ exit region is indicative of

potentially favorable regions for jet-forced ascent and associated air mass destabilization below.

From 4) it is evident (especially when a loop is created) that a series of disturbances have

dropped southeastward across the Great Basin and the Southwest before turning eastward toward Texas. At 0000 UTC 27 March, a weak leading subsynoptic-scale trough/cutoff cyclone over parts of Texas, Arkansas and Louisiana is in the process of being kicked out by a strong trailing trough approaching the coast of northern California. The Texas trough is associated with recurring severe weather over the southern Plains. By 0000 UTC 28 March, the leading and rapidly weakening subsynoptic scale trough/cutoff cyclone, now over the lower Mississippi River Valley, is no longer associated with severe weather. However, the trailing trough, which continues to intensify as it reaches the desert Southwest, helps to ignite a new severe weather outbreak much closer to the Rockies from western Texas northward to Oklahoma, extreme eastern Colorado and western Kansas and western Nebraska.. Finally, by 0000 UTC 29 March the trailing trough is crossing the southern Rockies and is itself associated with scattered severe storms over much of central and eastern Texas and Oklahoma.

From 5) it is apparent that the corridor of persistent southerly flow from the Gulf of Mexico and

associated lower tropospheric warm-air advection strengthens modestly and shifts eastward with the passage of the leading weaker trough/cutoff cyclone. The southerly flow in this corridor weakens briefly before it strengthens again significantly as it shifts back westward to a position close to the Rockies. This

westward shift of the southerly wind corridor occurs and associated increase in wind speeds occurs as

surface pressures fall more rapidly from eastern New Mexico northward to eastern Wyoming and western

Nebraska relative to surface pressures farther to the east. The resulting increasing in the isallobaric

gradient enables surface winds to back close to the Rockies ahead of the stronger trailing trough that is

approaching and crossing the Rockies, leading to the creation of an environment known to be favorable for the occurrence of severe weather over the southern and central Plains.

A few science issues.......

1) An interesting climatological study would be to determine where severe weather outbreaks occur relative to the location of the trough axis and the region of upward increase in cyclonic vorticity (or better to first approximation advection of relative vorticity by the thermal wind) as troughs move across the Great Basin, Intermountain West, the Rockies and the High Plains. A hypothesis is offered that severe convection would tend to occur further eastward relative to the trough axis over the High Plains than over the Great Basin and Intermountain West. The hypothesis rest on the idea that it is likely to be easier for air parcels to reach their LFC ahead of the advancing trough at the leading edge of the region of cyclonic

vorticity advection by the thermal wind where the air in the planetary boundary layer (PBL) is warmer,

more moist and more unstable than farther west where the air in the PBL is cooler, less moist and less

unstable. In the latter situation individual air parcels have to be lifted further to reach their LFC and,

as a result, deep convection is likely to occur closer to the core of the trough axis and colder air aloft

where ascent is sufficiently vigorous to lift air parcels through a deeper layer of the atmosphere.

2) Large-scale flow regimes that favor the passage of episodic deep, cold troughs from the Great Basin

eastward to the High Plains in spring play a very important role in determining overall rainfall amounts

and the spatial distribution of the rainfall. Investigations of possible future rainfall scenarios in a

warmer climate over the Southwest will have to consider how the frequency, intensity and pathways of these upper-level disturbances that are responsible for the bulk of the rainfall might change if there is any

hope of anticipating the future rain fall climatology over the Southwest. Downstream implications will

also need to be considered. Persistent ridging over the Southeast throughout much of March has resulted in widespread warm and dry conditions with stratospheric pollen levels that are....well....nothing to sneeze

at.

.........................................

The next Friday map discussion will be after spring break on the 13th. Paging Jason....

Lance