Fig. 1-2: A cooling stratosphere across North America (day-6 and day-10 ahead indicated) supports a less cold forecast in the medium-range.
Discussion: Medium-range forecasts are trending warmer across North America and especially the U.S. The catalyst to this warmer trend is a cold stratosphere over the polar region stretching south across Canada as indicated by the GFS at day-5 and day-10 (Fig. 1-2). As a review, the cooling and constricting stratosphere is compensated for by the warming troposphere below (where weather occurs). Consequently, the upper-level pattern in the troposphere creating our weather is less able to produce cold air and arctic air generation is cutoff. Arctic air already present will retreat to deep snow cover.
The midday GFS U.S. population weight HDD forecast is certainly compliant with the warmer trend as defined above (Fig. 3). The previous weekend forecasts were near or warmer than the 10-year normal. Climate signals (-NAO/-AO) continue to support a colder/stormy pattern but the stratospheric cooling and in-part warm mid-latitude SSTA weaken that effect. The Climate Impact Company “preferred” temperature anomaly medium-range forecast for North America reveals a much warmer look for the U.S. in the 6-10-day period (Fig. 4) followed by a marginally cold 11-15-day forecast (Fig. 5).
Fig. 3: U.S. gas population weight HDD forecast utilizing all models, their consensus and comparison with 24 hours ago and the 10-year/30-year normal. The midday 12Z GFS is added.
Fig. 4-5: The Climate Impact Company preferred 6-10/11-15-day temperature anomaly forecasts across North America.
The warmer theme to the outlook (mostly across the eastern half of the nation) also has a wetter character. The 12Z GFS identifies a swath of (all) rain from the Mid-south States, most focused on the Tennessee Valley and stretching to southern New England beginning later tomorrow and lasting into Saturday (Fig. 6). The heaviest rain is forecast across Kentucky and Tennessee where 3-5 in. is possible. Thunderstorm risk is included. The storminess will reach the northern Mid-Atlantic/southern New England coastal area late this week. However, the western North Atlantic surface is warmer than normal, and this storm will produce all rain except snow in mountain areas. Several inches of rain are possible in southern New England.
Fig. 6: The 12Z GFS 5-day precipitation amount forecast for the upcoming week across the East U.S.