Stratospheric Cooling, Alaska ridge bridge, and the December 2022 U.S. cold outbreak
Fig: 1: A cold stratosphere across Alaska on December 20th while compensating warming takes place across the East U.S.
Discussion: Monitoring of the stratospheric temperature patterns during the winter season is essential to predict evolution of extreme thermal regimes. Of primary interest is sudden stratospheric warming (SSW) episodes which are often linked to arctic air generation at ground level increasing risk of an outbreak of severe cold to high population middle latitudes.
During much of December, the opposite condition – stratospheric cooling – was dominant across west/northwest Canada and poleward. However, the broad area of stratospheric cooling shifted to Alaska and intensified on December 20th (Fig. 1). To compensate for the cooling, stratospheric warming emerged and peaked in intensity over the Northeast U.S. at that time. Beneath these extreme stratospheric thermal regimes, the troposphere warmed with associated amplified high pressure over Alaska (Fig. 2) stretching toward Siberia (often referred to as the “ridge bridge”). The ridge bridge pattern can tap arctic cold in Siberia and force cross-polar transit of the extreme chill into North America. Meanwhile, beneath the stratospheric warming, a polar vortex was emerging in Southwest Canada.
Today’s GFS upper air forecast identifies the final results of the described stratospheric condition of the past few days. The Alaska ridge bridge amplifies cross-polar to Siberia and clearly motivates cross polar arctic air flow (Fig. 3) into the emerging polar vortex over the Midwest U.S. today. The combination of the cross polar arctic air flow and emerging polar vortex cause an arctic outbreak into the U.S. featuring afternoon temperatures today 40F to 45F below normal in Kentucky (Fig. 4).
Fig: 2: Beneath the stratospheric cooling over Alaska, the troposphere beneath warms and a “ridge bridge” high pressure area forms. Just downstream, associated with stratospheric warming, the onset of a polar vortex in Southwest Canada is forming on December 20th.
Fig: 3: The GFS 500 MB anomaly forecast across the northern hemisphere for December 23, 2022 identifying the fast-emerging polar vortex over the Midwest U.S. and “ridge bridge” over and north of Alaska.
Fig: 4: GFS projects the coldest temperature anomalies for the December 2022 Arctic Outbreak for 4PM EST December 23, 2022.