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Observations for Jan 24/25 snowstorm


HoarfrostHubb

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Kick-off January 23, 2015

Weather advisory: Mesoscale Discussion #39

MESOSCALE DISCUSSION 0039

NWS STORM PREDICTION CENTER NORMAN OK

1048 PM CST FRI JAN 23 2015

Areas affected, portions of cntrl and ern pa, cntrl/nrn nj, SE ny

including nyc and long island, portions of ct

concerning, heavy snow

valid 240448Z - 241045Z

summary, heavy snow will be developing overnight tonight --

especially from 06Z to 09Z -- from portions of the cntrl

appalachians enewd to srn new england and the long island

coast, including the new york city metropolitan area. local

snowfall rates to 1 inch per hour will be possible.

discussion, mosaic radar imagery indicates a shield of widespread

precipitation extending from the mid-atlantic to the ohio

valley, embedded within a broad warm-advection zone analyzed at

h85. this precipitation will continue to spread enewd through the

night, as related isentropic ascent is maintained well downstream

of a positively tilted midlevel trough, which water vapor loops

depict as extending from the middle oh valley to the lower ms

valley. recent sfc observations suggest 2-hourly pressure falls

around 3.5-4.5 mb from sern va to ern nc, most likely a reflection

of increasing large-scale ascent preceding the trough becoming

juxtaposed with a ne/sw-oriented baroclinic zone/inverted trough

analyzed roughly 40-75 miles inland from the coast. sfc low pressure

is forecast to consolidate/deepen from this interaction and move off

the coast, with a corresponding increase in waa and frontogenetic

ascent to its n. in turn, present precipitation intensity will be

maintained, or perhaps increase, across the discussion

area, particularly as a deformation band becomes established well

n/nw of the evolving sfc cyclone.

subjective spatial interpolation between the 00Z iad and okx raobs

suggests near- to below-freezing temperatures existing, and

continuing to exist, throughout the troposphere across the mcd

area. this is especially the case given continued nocturnal diabatic

cooling and modest adiabatic cooling associated with dcva preceding

the approaching midlevel trough. where sfc temperatures remain

slightly above 0C from sern pa to cntrl nj, wet-bulb-zero heights

will generally remain aob 500 ft agl -- as affirmed by rap forecast

soundings and interpolated observed soundings. as such, snow is

forecast to be the predominant precipitation type across the

discussion area, with deep, near-isothermal/near-0-c layers

fostering efficient growth of snow aggregates. as such, snowfall

rates will locally reach 1 inch per hour. such rates in heavy snow

will likely materialize between 06Z and 09Z from cntrl pa to cntrl

nj, while spreading enewd toward the new york city area and srn new

england through the late-night hours.

very late tonight, generally after 10Z, it is possible that

strengthening onshore flow along the e-cntrl nj coast -- in response

to cyclogenesis farther south -- may support sufficient low-level

warming for precipitation to mix with and/or change to rain.

however, this will generally be in a confined area near the coast

where oceanic heat fluxes are maintained inland only over a modest

displacement.

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