Global Atmospheric Angular Momentum Shift to Positive Phase

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Fig. 1-2: GFS ENS/ECM ENS are agreeable on a nationally colder than normal Day-15 (March 12th) temperature anomaly forecast.

Discussion: Meteorological winter 2022-23 produced two extremes across the U.S. including a wintery cold and snowy regime across the western half of the nation while the eastern half was unusually mild. However, both the GFS ENS and ECM ENS are agreeable to a nationally colder than normal regime in 15 days (Fig. 1-2). During that time, the western chill is enhanced by widespread deep snow cover and possibly some arctic air while that cold air source region is encouraged to shift into the East by a farther south jet stream pattern.

The slight southern shift of the jet stream with a strong zonal (west-to-east) character allows cold air to drain southward out of Canada and expand from coast-to-coast. The farther south jet stream pattern is hinted at by a sudden emerging strong positive phase of global atmospheric momentum (+GLAAM) normally present when mid-latitude jet streams are stronger than normal (Fig. 3).

The result of the GLAAM strengthening is a colder U.S. gas population weight HDD forecast. Since Friday, the forecast trend of the consensus of all models is higher U.S. heating demand for the first two full weeks of March to above both the 30-year and 10-year normal (Fig. 4). Interestingly, the GFS ENS and ECM ENS are the coldest models.

Fig. 3: Global atmospheric angular momentum was changeable during the northern hemisphere winter and is forecast to shift to positive phase in March.

Fig. 4: U.S. gas population weight HDD forecast using a consensus of all operational models and a comparison with 48 hours ago plus the 30-year/10-year normal.

Fig. 5: Latest NOAA/NWS weather watch, warning, and advisory areas.

During winter, most people like to see some snowfall. The West U.S. has observed what is becoming historic snowfall in many locations. Right now, a Blizzard Warning is in effect for the Sierra Nevada (Fig. 5). The type of heavy snow this region receives is extremely dangerous. Snowflakes are the size of an adult’s hand and during heavy snow there is no visibility. Outdoors is completely uninhabitable. The California snow has been relentless and mountain area snowpack is likely to climb to levels not seen in a generation.

Despite the immense winter precipitation across California including piling snows across mountain areas that extend to the Southern Great Basin (Fig. 6) the West U.S. Drought remains intact due to the long-term precipitation deficit. Extremely important during the coming weeks will be snowmelt forecasts to govern optimum effectiveness of water storage (collection).

The leading catalyst to define the California storminess is an immense negative phase of the Pacific North America (-PNA) index. The peak -PNA was over-the-weekend and attached to the snow warnings that reached the Los Angeles Basin. The 15-day PNA forecast reveals a very slow weakening of the negative phase which leaves California susceptible to more storminess through the first third of March.

Elsewhere, high wind stretches across West Texas to the East-central U.S. the next 2 days. While the West stays snowy, some snow is also likely for the northern Great Lakes to New England MON/TUE.

Fig. 6: Percent of normal snow-water-equivalent for regions of the West U.S. Dark blue represents values >200%. Green represents >150%. Note that Northwest U.S. is mostly drier.

The latest mega-cluster ensemble “most likely” temperature anomaly forecasts for the U.S. indicate the Northwest chill in the 6-10-day period (Fig. 7) expands eastward in the extended-range (Fig. 8) but without ferocity.

Fig. 7-8: Mega-cluster ensemble “most likely” temperature anomaly forecasts for the medium-range across the U.S.