Woznicki - Sterling, VA

Sterling, VA Forecast Area Valid 4/20/12

Current Analysis through 12 UTC 4/21/12:

An upper-level ridge is currently dominating the pattern over the area, and will still be the main influence through the morning hours tomorrow. Highs today should warm, in the lower to mid 70s, across much of central (north-south oriented) Virginia. This same area is actually staying slightly cooler due to an area of mid-level cloud cover - brought on by good moisture advection in the upper levels (700 mb and 500 mb.) Western Virginia, West Virginia, and Maryland may see slightly cooler temps today, especially in areas with higher elevation. Good warm-air advection ahead of the approaching cold front should keep temperatures near 60 overnight tonight, especially where the cloud cover persists. Cooler temperatures tonight should be confined to those ares that stay clear (until the frontal region approaches). Winds are fairly light of of the southwest for most of the area, so some upslope cooling may occur along the edges of the Appalachians on the Northwest fringes of the area. The 12 UTC sounding from this morning at Dulles International shows a good inversion at the surface beneath this high pressure.

Current Surface Obs and Radar

Current Visible Satellite

18 UTC 1000-500 mb Thickness and 6-hr Precip

18 UTC 700 mb Temp and Winds

12 UTC Dulles Sounding from this Morning

12 UTC 4/21 - 12 UTC 4/23:

A low-pressure system tracking far to the north of the area will bring an associated cold front extending southward. This front will begin to affect the western part of the area in the mid-day to afternoon hours tomorrow, starting from west to east. The GFS does seem to bring this front in a little faster than the NAM, but not enough of a difference to impact the larger-scale synthesis of the two low-pressure systems. There is also a good agreement with timing of the UKMET. Another trough of low-pressure that ejects out of the southern plains into the Gulf of Mexico will also be a big player in the development of precipitation over the area. Showers and a few thunderstorms will be possible as the front associated with the northern system approaches tomorrow. CAPE values will exceed 1000 J/kg, but the associated showers look to be much more on a synoptic scale rather than mesoscale/convective. Some strong winds and lightning will be the main storm threats. Very strong positive vorticity advection is evident as the souther low-pressure center tracks northeastward throughout the overnight hours on Saturday, and significant precipitation amounts will be experienced area-wide (very good omega values as well). Significant warm-air advection is also evident in the lower-levels in our area just north of the southern-low pressure system. Strong frontogenetical forcing will also be available as the magnitude of the temperature gradient drastically increases between the two low-pressure systems. Highest rainfall totals for this system look to be in the far southeast areas of the region at between 1-2 inches of rainfall, with lower values farther westward away from the strongest forcing. Very strong winds overnight on Sunday afternoon-overnight will cause high surf, and could warrant a small-craft advisory.

18 UTC CAPE Tomorrow

00 UTC 500 mb Vorticity for Monday

06 UTC Monday 850 mb Analysis

22 UTC Sunday Simulated Reflectivity

3-hr Precip ending 00 UTC Monday

12 UTC 4/23 - 12 UTC 4/25:

Most of the rain in the area looks to end by 12 UTC on Monday, but this is just the time that the deformation zone on the west-northwest side of the low-pressure system looks to start producing precipitation for the hill/mountain regions of West Virginia, Northern Virginia, and the western protrusion of Maryland. Soundings for the time period show temperatures a little too warm at the surface for snow in Washington, D.C, however, areas just west of here in higher terrain may see significant snowfall for an excess of 12 hours. Critical thicknesses for this area look to be favorable for snow. It looks like several inches of snow will fall over the mountainous regions of the area. Some warm-air advection is still evident with this system during the time of projected snowfall, but most of the other forcings for vertical motions are beginning to shut off. However, these forcings, such as vorticity and temperature advection, are less negative than the areas directly adjacent. This could help the snowfall persist for a long enough time to get significant totals. After the snowfall shuts off on Monday night, it looks like a warm/stationary front, associated with a low-pressure center off to the west, brings some more rain to the area throughout the remainder of the forecast time. Some storms may be possible in the warm sector, but the remaining cold air north of the warm front will inhibit any storm ingredients.

12 UTC 4/23 Precip Type

00 UTC 4/24 Precip Type

00 UTC 4/24 Sounding

09 UTC 4/25 850-700 mb Thickness