Sara Ganetis led a discussion titled, “Wonderful Weekend Weather, Anyone?”
The discussion began with an overview of the current animated water vapor imagery over the Northern Hemisphere. The imagery showed two areas of tropical activity with Tropical Storm (TS) Rachel in the Eastern Pacific and an area of convection which was currently being investigated by the National Hurricane Center (NHC) in the Atlantic. Both systems are not forecast to strengthen much more than their current states due to movement into regions of cooler sea surface temperatures (SSTs) and increased wind shear. The imagery also showed the system that moved through the area yesterday, 25 September 2014, and the clear conditions associated with the ridge and high pressure building in behind it to the west.
The weak coastal low that brought precipitation to the area yesterday was discussed, especially because the GFS operational model had an area of high pressure anchored over the Northeast U.S. until about 4.5 days lead time. Portions of central and Eastern Long Island received > 1.00 in of rain. An interesting observation was made using the vertically pointing radar on top of one of the buildings at our campus (Endeavour Hall)– it actually snowed! Well, it snowed above approximately 3 km but then melted into rain before reaching the ground. The arrows show the melting layer meaning that it was snowing above that and raining below. The signature shows up in the fall speed plots by a large gradient between slowly falling snow that melts into rain and falls faster. The bright banding in the reflectivity signature occurs when the snow particles get coated with liquid water as they melt and reflect a lot more radiation back to the radar than the snow/ice and even pure raindrops. So as a sanity check, I also looked at the corresponding 12Z 25 September 2014 sounding from OKX which shows the environment becoming subfreezing above 3 km which makes sense given what we see with the MRR images. Sorry for nerding out– I’m just super into the “cooler” aspects of weather. If you ever want to check out the MRR in realtime, Mark Lang put together this page– http://wx.somassbu.org/pages/
Further examination of the event showed that a shortwave trough in the Midwest seemingly materialized between operational runs of the GFS and, although the forcing was weak, was sufficient to produce the weak coastal low. An interesting discussion ensued based on the in-house operational WRF models initialized with both the GFS and NAM. Comparing the 72-h forecast verifying at 12Z 25 September, the NAM-WRF had spun up a much stronger cyclone (997 mb versus the GFS-WRF’s 1001 mb low). Digging deeper, we discovered that the NAM-WRF, while starting out similar to the GFS-WRF in terms of an upper-level vorticity filament digging down from the Midwest, cutting off and spinning up the coastal low actually converged with some vorticity of tropical origin coming north up the coast from South Carolina. Further examination showed that this low actually exhibited hurricane-force winds along the coast of North Carolina and was warm-core. Therefore, the NAM-WRF spun up a hurricane instead of a weaker coastal low! Oops? This discussion was very interesting, though, and motivated investigation of tuning the operational models to not be so sensitive to surface fluxes from the warm, ocean waters to prevent any future fake-hurricane development.
The Fall foliage reports and outlook were reviewed to show that nowhere in the Northeast has peaked but areas in northern Maine and the higher terrain of the Adirondacks upstate are exhibiting “High” color, which is just shy of peak. A discussion of what it takes to create the most vibrant foliage was motivated by an image created by the NWS La Crosse, WI office. Cool, but above-freezing nights following warm sunny days are necessary for the most sugars to be created within the leaves during the day and then if remained unfrozen will be kept within the veins of the leaves and result in vibrant pigments (USDA Forest Service). The forecast through the weekend throughout the Northeast is calling for clear and warm sunny days (highs approaching 80 F in both locations both Saturday and Sunday) and cool, above-freezing nights (lows remaining > 40 F). Book your leaf-peeping trip now and enjoy the wonderful weekend weather (hurricane-free)!