3-4 August 2008 Hudson/Mohawk Convergence

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Hudson/Mohawk convergence is typically thought to occur mostly during the cold season, often contributing to lingering snow and low clouds within a 3 mile radius of Albany, NY. However, Hudson/Mohawk Convergence can occur any time of year, including the warm season. However, the effects during the warm season are considerably different than during the cold season. The surface and upper-level features are generally the same, regardless of the season, but the sensible weather is what is different.

 

On 3-4 August 2008, Hudson/Mohawk Convergence occurred, that supported a localized band of low clouds, showers and thunderstorms, which in some cases affected the Albany airport, and produced locally heavy rains. The low clouds, showers and thunderstorms were not well resolved in forecast guidance, including the GFS, NAM, WRF and MOS. However, there were signals in the data throughout the mornings of 3 and 4 August that proved helpful in anticipating Hudson/Mohawk Convergence for later in each day.

 

a) b) c)

Figure 1. Upper plots from the Storm Prediction Center at 850 hPa from a) 1200 UTC 3 August b) 0000 UTC August 4, and c) 1200 UTC August 4. Note the low pressure slowly exiting the northeastern U.S. and the lingering trough axis over the interior northeastern U.S.

 

a) b) c)

Figure 2. Upper plots from the Storm Prediction Center at 500 hPa from a) 1200 UTC 3 August b) 0000 UTC August 4, and c) 1200 UTC August 4. Note the upper-level low pressure slowly exiting the northeastern U.S. and the lingering upper trough axis over the interior northeastern U.S.

 

a) b) c)

 

Figure 3. Raobs at 1200 UTC 3 August from a) Albany, NY, b) Buffalo, NY and c) Manawaki, ON. Note the near saturation below 700 hPa in each sounding, and although relatively limited, surface-based instability exists.

 

a) b) c)

 

Figure 4. Raobs at 1200 UTC 4 August from a) Albany, NY, b) Buffalo, NY and c) Manawaki, ON. Note the continued saturation below 700 hPa, but drying is gradually filtering in at mid and upper levels.

 

a) b)

 

Figure 5. Raobs at 0000 UTC 4 August from a) Albany, NY, b) Manawaki, ON. Note the continued saturation below 700 hPa, but drying is gradually filtering in at mid and upper levels. Convective instability is still present.

 

a) b)

 

Figure 6. Surface plots from a) MSAS 1800Z 3 August MSLP and wind barbs, and b) MSAS 1900Z 3 August MSLP and station plot. Note the SW/NE-oriented isobars across eastern NY. Also note the northeast winds along the NY/VT border, and NW winds over central and southern NY into southern New England. This provided broad convergence across the region that could become enhanced surface convergence as winds channeled down the Mohawk and Hudson Rivers into the Capital District.

 

a) b)

 

Figure 7. Visible satellite imagery from a) 2145 UTC 3 August and b) 2145 UTC 4 August. Note the lines of convective cloudiness oriented NNW/SSE across eastern NY on the 2 successive days.

 

 

Figure 8. KENX loop of 0.5 base reflectivity in hourly increments from 2002Z through 2300Z 3 August. Note the showers and thunderstorms in NNW/SSE oriented lines training over roughly the same locations.

 

a) b)

 

Figure 9. Regional 0.5 base reflectivity at a) 2200 UTC 4 August and b) 2257 UTC 4 August. Note the showers and thunderstorms in NNW/SSE oriented lines training over roughly the same locations for the second day.

 

 

Figure 10. METAR observations at Albany, NY airport from 1747Z through 2351Z 3 August. Note the thunderstorms reported and the brief IFR visibility from 2309Z to 2316Z. Rainfall of 0.12 was observed at the airport, but some locations within 5 miles of the airport received over 1.00 of rain.