Really good write up from LWX regarding subtle pattern changes that have led to less phasing in recent runs. Threading the needle rarely works in everyone’s favor. .LONG TERM /WEDNESDAY THROUGH SATURDAY/... ...Looming Wednesday-Thursday Winter Storm Threat Yet to be Fully Resolved... The focus of the long term period continues to be on an East Coast winter storm threat and its potential impacts to the Mid- Atlantic region. For the better part of the last week, ensembles have been hinting at the potential of significant snow around February 20, though there are some trends in the last 48 to 72 hours worth noting. For mid week, upper-level pattern/ensemble mean 500 hPa height trends the last couple of days have been for (1) a somewhat weaker northern stream upper low descending into the northern Plains, and (2) a weaker West Coast/Rockies ridge likely due at least in part to a stronger and faster shortwave progged to be approaching the Pacific Northwest. It appears these trends reached a critical threshold in the 15/12Z guidance suite, with a bimodal distribution among varying ensembles slightly favoring less meridional flow and a more offshore surface low track. This pattern shift, if it verifies, would allow the southern stream shortwave to not be fully caught as quickly by the slightly less amplified northern stream upper low, with less phasing resulting in the further southeast/offshore track. The trend was most obvious in the EPS (ECMWF ensemble) and MOGREPS-G (UKMET ensemble), which showed a southeastward shift in the axis of highest snowfall probabilities. The trend persisted through the 15/18Z cycle, and appears to have stabilized a bit in the 16/00Z cycle (though with some -increasing- spread). Interestingly, some machine learning guidance (i.e. the GraphCast GFS and ECMWF-AIFS) had been showing this offshore track potential for several days now. However, their latest runs have trended a bit to the northwest (closer to the coast), perhaps indicative of a stabilizing or even reversal in trends. Also of note, although the highest probabilities for 6- and 12-inch plus snows have shifted southeastward for the time being, probabilities for 2 inches of snow remain elevated. This suggests that at least some snow is highly likely (probably with the passing northern stream upper trough/low even in a less amplified scenario). It is important to note this trend, but also the continued variability and likely future shifts in the track. Despite the shift in ensemble means and the rather low amounts of snow locally in the 16/00Z CMC and UKMET deterministic, the ECMWF deterministic remains steadfast in solid warning level snow for much of the region. The recent GFS and ICON deterministic also indicate a high likelihood of warning level snow for at least roughly the southeastern half of the CWA. Given the amount of cold air, moisture, and energy present, the potential for a significant winter storm still exists and needs to be closely monitored. But, the exact details of how this all unfolds remain unclear this far in advance. .
